1 00:00:16,015 --> 00:00:18,385 Let's get started. 2 00:00:18,385 --> 00:00:22,522 We're going to keep talking about polymers today. 3 00:00:22,522 --> 00:00:26,693 Polymers-- that's also going to be the topic of the quiz. 4 00:00:26,693 --> 00:00:32,198 And because of the exam and everything-- and so 5 00:00:32,198 --> 00:00:34,934 we're having our second lecture on polymers today. 6 00:00:34,934 --> 00:00:37,804 The quiz tomorrow will be at the end of recitation 7 00:00:37,804 --> 00:00:40,640 so that you can have a full second recitation 8 00:00:40,640 --> 00:00:42,909 on this topic. 9 00:00:42,909 --> 00:00:43,410 OK. 10 00:00:46,379 --> 00:00:47,814 I mentioned this on Monday, right? 11 00:00:47,814 --> 00:00:50,483 Monday we introduced polymers and I 12 00:00:50,483 --> 00:00:54,254 covered two different ways to make polymers, OK? 13 00:00:56,990 --> 00:00:58,825 Let's write those down. 14 00:00:58,825 --> 00:01:03,430 Let's write those down with chalk, which is somewhere. 15 00:01:03,430 --> 00:01:06,299 Here we go. 16 00:01:06,299 --> 00:01:15,909 On the one hand, we had what we called radical initiation. 17 00:01:15,909 --> 00:01:21,146 So we use a radical initiator. 18 00:01:21,146 --> 00:01:22,649 Remember this R dot? 19 00:01:22,649 --> 00:01:25,652 And we talked about radicals, OK? 20 00:01:25,652 --> 00:01:29,722 And how that can lead to this kind of chain reaction. 21 00:01:29,722 --> 00:01:30,223 Right? 22 00:01:30,223 --> 00:01:35,395 And so this is called chain polymerization. 23 00:01:35,395 --> 00:01:39,065 Sometimes it's called addition, polymerization. 24 00:01:39,065 --> 00:01:43,570 But it all involves a radical to start it 25 00:01:43,570 --> 00:01:45,338 and then a double bond. 26 00:01:45,338 --> 00:01:47,140 Remember, we talked about that double bond. 27 00:01:47,140 --> 00:01:50,043 The importance of the double bond. 28 00:01:50,043 --> 00:01:50,543 Right. 29 00:01:50,543 --> 00:01:53,146 So this would have some monomer. 30 00:01:53,146 --> 00:01:55,448 This would have some monomer. 31 00:01:58,751 --> 00:02:03,156 But it's got to have with a double bond. 32 00:02:03,156 --> 00:02:05,158 That's not an equal sign, that's a double bond. 33 00:02:05,158 --> 00:02:06,926 Now, the second way we talked about 34 00:02:06,926 --> 00:02:09,529 was called condensation polymerization. 35 00:02:09,529 --> 00:02:14,267 So condensation polymerization these 36 00:02:14,267 --> 00:02:21,407 are two ways we covered that allow you to make these really, 37 00:02:21,407 --> 00:02:23,209 really long chains. 38 00:02:23,209 --> 00:02:26,312 That's what a polymer is, right? 39 00:02:26,312 --> 00:02:29,482 Polymer-- "mer" being from say hundreds 40 00:02:29,482 --> 00:02:33,386 to thousands and even millions. 41 00:02:33,386 --> 00:02:35,855 And in condensation polymerization, 42 00:02:35,855 --> 00:02:40,793 you would have two different-- you could even have more. 43 00:02:40,793 --> 00:02:41,494 --monomers. 44 00:02:44,930 --> 00:02:47,767 And these two different monomers, well, 45 00:02:47,767 --> 00:02:48,835 let's write it explicitly. 46 00:02:48,835 --> 00:02:51,337 They react, OK? 47 00:02:51,337 --> 00:02:55,341 And they react because they've got these end groups 48 00:02:55,341 --> 00:02:58,311 that when they see each other, right, 49 00:02:58,311 --> 00:03:03,283 from one monomer to the next, they form a bond. 50 00:03:03,283 --> 00:03:05,919 And in the case that we did on Monday, 51 00:03:05,919 --> 00:03:08,721 they formed an amide bond. 52 00:03:08,721 --> 00:03:10,056 And they gave off water, right? 53 00:03:10,056 --> 00:03:12,125 So water left. 54 00:03:12,125 --> 00:03:14,594 So when you do condensation polymerization, 55 00:03:14,594 --> 00:03:16,396 you start with two different monomers. 56 00:03:16,396 --> 00:03:18,064 They've got the end groups on them 57 00:03:18,064 --> 00:03:20,133 that when they see each other, they want to react. 58 00:03:20,133 --> 00:03:25,004 You wind up with a polymer that weighs a little bit less 59 00:03:25,004 --> 00:03:26,639 than the two monomers, right? 60 00:03:26,639 --> 00:03:29,509 Because you've given off water. 61 00:03:29,509 --> 00:03:31,010 You could give off other things. 62 00:03:31,010 --> 00:03:32,111 There's a lot of examples. 63 00:03:32,111 --> 00:03:38,117 So in the reaction you give off maybe H2O, like we did. 64 00:03:38,117 --> 00:03:42,055 Could be HCL. 65 00:03:42,055 --> 00:03:43,022 Could be NH3. 66 00:03:43,022 --> 00:03:44,224 There are a number of ways. 67 00:03:44,224 --> 00:03:45,692 I don't want you to get the feeling 68 00:03:45,692 --> 00:03:48,561 that the only way you can do condensation polymerization 69 00:03:48,561 --> 00:03:51,631 is the nylon example, all right? 70 00:03:51,631 --> 00:03:54,634 There's a lot of ways you can do this. 71 00:03:54,634 --> 00:03:56,668 But one thing that is certainly true 72 00:03:56,668 --> 00:04:01,674 is that in forming that bond, you give something else up. 73 00:04:01,674 --> 00:04:05,445 And that's why sometimes people like calling this addition. 74 00:04:05,445 --> 00:04:07,480 Because in addition polymerization, 75 00:04:07,480 --> 00:04:12,619 right, you take a monomer and you just keep adding it. 76 00:04:12,619 --> 00:04:16,723 And the sum is n times the monomer, right? 77 00:04:16,723 --> 00:04:18,124 The sum of the weight. 78 00:04:18,124 --> 00:04:20,326 Whereas in this case, it's a little bit less 79 00:04:20,326 --> 00:04:22,862 because you did a reaction. 80 00:04:22,862 --> 00:04:23,763 OK. 81 00:04:23,763 --> 00:04:25,832 So that's all kind of getting us back in the mood. 82 00:04:25,832 --> 00:04:28,501 That's what we did on Monday. 83 00:04:28,501 --> 00:04:32,905 Now today, I want to talk about the properties of polymers 84 00:04:32,905 --> 00:04:33,940 and how we control them. 85 00:04:33,940 --> 00:04:34,907 How we can change them. 86 00:04:34,907 --> 00:04:36,109 Different ways that we can change them. 87 00:04:36,109 --> 00:04:37,644 I can't cover everything but I want 88 00:04:37,644 --> 00:04:40,880 to cover some key elements of engineering polymers. 89 00:04:40,880 --> 00:04:42,682 So we're going to talk about the properties 90 00:04:42,682 --> 00:04:46,753 and a couple of specific things that we use to control them. 91 00:04:46,753 --> 00:04:51,457 So let's start a list here in the middle. 92 00:04:51,457 --> 00:04:55,128 And the first thing is something we already 93 00:04:55,128 --> 00:04:58,431 did talk about Monday, which is just the monomer itself, right? 94 00:04:58,431 --> 00:05:00,933 So what is it that matters in a polymer? 95 00:05:00,933 --> 00:05:06,539 What is it that dictates what a polymer will be like, right? 96 00:05:06,539 --> 00:05:09,942 Well, one is the monomer. 97 00:05:09,942 --> 00:05:11,711 So that's the unit-- 98 00:05:11,711 --> 00:05:12,378 well, OK. 99 00:05:12,378 --> 00:05:14,047 We just said that you need two here. 100 00:05:14,047 --> 00:05:16,983 So maybe there's monomers. 101 00:05:16,983 --> 00:05:18,751 But that's the unit that you're repeating. 102 00:05:18,751 --> 00:05:20,953 Clearly that's going to be really important, right? 103 00:05:20,953 --> 00:05:23,089 What did you put there, right? 104 00:05:23,089 --> 00:05:25,224 Is it C2H4? 105 00:05:25,224 --> 00:05:27,026 Is it polyethylene or is it something else? 106 00:05:27,026 --> 00:05:28,194 Did you add something to it? 107 00:05:28,194 --> 00:05:30,863 The benzene ring, right? 108 00:05:30,863 --> 00:05:33,099 And so that's going to be really important. 109 00:05:33,099 --> 00:05:34,667 The other thing that we talked about-- 110 00:05:34,667 --> 00:05:36,436 and so we're going to create this list --is 111 00:05:36,436 --> 00:05:37,503 the molecular weight. 112 00:05:40,173 --> 00:05:44,043 Now remember, molecular weight. 113 00:05:44,043 --> 00:05:46,946 Oh we, like every little community, 114 00:05:46,946 --> 00:05:48,781 likes to come up with its own names. 115 00:05:48,781 --> 00:05:50,650 We know grams per mole. 116 00:05:50,650 --> 00:05:54,120 And then they say, oh that's a dalton. 117 00:05:54,120 --> 00:05:56,989 And so now we have to say, OK, a dalton is a gram per mole. 118 00:05:56,989 --> 00:06:00,693 Kilodalton would be 1,000 grams per mole. 119 00:06:00,693 --> 00:06:03,396 But we also said that this gives us 120 00:06:03,396 --> 00:06:06,165 the degree of polymerization. 121 00:06:06,165 --> 00:06:07,567 So that's really important, right? 122 00:06:07,567 --> 00:06:12,872 Because how much to the degree of polarization-- 123 00:06:12,872 --> 00:06:17,810 I mean, if I know what my mer is and I 124 00:06:17,810 --> 00:06:22,248 know the molecular weight, the grams per mole of a strand, 125 00:06:22,248 --> 00:06:24,450 then I know how many mers went into it. 126 00:06:24,450 --> 00:06:26,152 So I know the degree of polymerization. 127 00:06:26,152 --> 00:06:27,887 That's just n, right? 128 00:06:27,887 --> 00:06:29,355 It's some number. 129 00:06:29,355 --> 00:06:35,094 But this is important and I'll put it over here 130 00:06:35,094 --> 00:06:37,196 so I can keep going down on that board. 131 00:06:37,196 --> 00:06:41,933 When you have a synthesis of a polymer, 132 00:06:41,933 --> 00:06:44,070 doesn't matter which one of those you use, 133 00:06:44,070 --> 00:06:46,606 you're going out 100,000 units. 134 00:06:46,606 --> 00:06:48,908 Yeah, I can't say well, every single one 135 00:06:48,908 --> 00:06:49,976 is going to be 100,000. 136 00:06:49,976 --> 00:06:51,544 That is really, really hard to do. 137 00:06:51,544 --> 00:06:53,679 So instead you get a distribution. 138 00:06:53,679 --> 00:06:55,615 So when you say the molecular weight, what you 139 00:06:55,615 --> 00:06:58,683 mean is some average, right? 140 00:06:58,683 --> 00:07:01,821 So this would be like maybe the molecular weight, maybe 141 00:07:01,821 --> 00:07:05,158 this is in daltons, maybe it's in kilodaltons. 142 00:07:05,158 --> 00:07:09,395 And you say, aw, the molecular weight is 40 kilodaltons. 143 00:07:09,395 --> 00:07:11,964 No, that's the average. 144 00:07:11,964 --> 00:07:14,934 You might have ones that are way, way longer out here. 145 00:07:14,934 --> 00:07:17,336 Maybe orders of magnitude longer. 146 00:07:17,336 --> 00:07:22,141 This distribution can be orders of magnitude. 147 00:07:28,047 --> 00:07:29,615 But it still can be you know, you 148 00:07:29,615 --> 00:07:32,118 can try to make them longer so you have a distribution that's 149 00:07:32,118 --> 00:07:32,784 more out here. 150 00:07:32,784 --> 00:07:34,120 Or you make them shorter, right? 151 00:07:34,120 --> 00:07:36,289 So this is something that is important 152 00:07:36,289 --> 00:07:37,723 and that can be engineered. 153 00:07:37,723 --> 00:07:40,760 You can imagine if my chains are really, really long, 154 00:07:40,760 --> 00:07:43,563 then maybe it's a tougher material, right? 155 00:07:43,563 --> 00:07:47,233 Because there's just more covalently bonded backbone 156 00:07:47,233 --> 00:07:49,001 to break, right? 157 00:07:49,001 --> 00:07:52,305 You can imagine that that might be one thing. 158 00:07:52,305 --> 00:07:55,341 Oh, but the thing that we gotta talk about 159 00:07:55,341 --> 00:07:59,011 is the chemistry which dictates the bonding. 160 00:08:01,914 --> 00:08:03,749 OK, so the chemistry. 161 00:08:03,749 --> 00:08:05,651 So the interactions. 162 00:08:05,651 --> 00:08:10,523 And this takes us back to so many happy moments 163 00:08:10,523 --> 00:08:14,560 from this fall where we talked about intermolecular forces. 164 00:08:14,560 --> 00:08:17,463 All of those apply to polymers. 165 00:08:17,463 --> 00:08:19,065 They all apply. 166 00:08:19,065 --> 00:08:20,032 What are the IMFs? 167 00:08:23,369 --> 00:08:26,906 Well, if I gave you an example, like a question-- 168 00:08:26,906 --> 00:08:30,209 OK, maybe you could say, well, let's see-- 169 00:08:30,209 --> 00:08:32,211 OK, I've got polyethylene. 170 00:08:38,584 --> 00:08:40,486 Remember, this how we write it? 171 00:08:40,486 --> 00:08:42,755 And in this case, I've left off my Hs. 172 00:08:42,755 --> 00:08:44,457 But the little sticks there means there's 173 00:08:44,457 --> 00:08:47,193 an H above and below each one. 174 00:08:47,193 --> 00:08:49,295 I've got polystyrene. 175 00:08:49,295 --> 00:08:50,663 So that's polyethylene. 176 00:08:50,663 --> 00:08:53,833 Here's polystyrene. 177 00:08:53,833 --> 00:08:57,270 Remember, this one has a benzene ring. 178 00:08:57,270 --> 00:09:00,206 Oh, that's a terrible drawing of a benzene ring. 179 00:09:00,206 --> 00:09:02,608 But it's what it's supposed to be. 180 00:09:02,608 --> 00:09:05,945 They have to come out of the parentheses. 181 00:09:05,945 --> 00:09:07,647 If it doesn't come out of the parentheses 182 00:09:07,647 --> 00:09:11,551 is not a polymer because you're not indicating repetition, 183 00:09:11,551 --> 00:09:12,418 right? 184 00:09:12,418 --> 00:09:15,087 OK, that's supposed to be benzene. 185 00:09:15,087 --> 00:09:18,958 And that'll be polystyrene. 186 00:09:18,958 --> 00:09:21,627 And then how about another one? 187 00:09:21,627 --> 00:09:22,962 I'll put it here. 188 00:09:22,962 --> 00:09:25,898 How about polyvinyl alcohol? 189 00:09:25,898 --> 00:09:27,533 So now watch this. 190 00:09:27,533 --> 00:09:32,305 I'm going brackets because remember, it's OK. 191 00:09:32,305 --> 00:09:36,876 And so here OH, right? 192 00:09:36,876 --> 00:09:42,014 And each one of these would have some number of repeats. 193 00:09:42,014 --> 00:09:45,318 That's polyvinyl alcohol. 194 00:09:45,318 --> 00:09:49,288 I can say well, just look at those monomers. 195 00:09:49,288 --> 00:09:51,924 Just look at the mers, right? 196 00:09:51,924 --> 00:09:54,594 Remember, the monomers if these were 197 00:09:54,594 --> 00:09:57,563 made with chain polarization, the monomers would 198 00:09:57,563 --> 00:09:58,831 have a double bond, right? 199 00:09:58,831 --> 00:10:00,132 Remember that. 200 00:10:00,132 --> 00:10:03,269 But so this is the repeat unit in the polymer. 201 00:10:03,269 --> 00:10:06,739 That's what goes into the brackets or the parentheses. 202 00:10:06,739 --> 00:10:09,442 I can look at it and say, which one of these 203 00:10:09,442 --> 00:10:13,846 is going to be the strongest mechanically? 204 00:10:13,846 --> 00:10:17,750 And you know from your IMF days. 205 00:10:17,750 --> 00:10:20,853 You know which one it is because only one of them 206 00:10:20,853 --> 00:10:23,623 can form a stronger bond than London. 207 00:10:23,623 --> 00:10:25,157 They're all going to have London. 208 00:10:25,157 --> 00:10:28,060 Everyone has London. 209 00:10:28,060 --> 00:10:31,330 But this one has hydrogen as well. 210 00:10:31,330 --> 00:10:32,498 That's got hydrogen bonds. 211 00:10:32,498 --> 00:10:37,036 So the polyvinyl alcohol will be stronger, right? 212 00:10:37,036 --> 00:10:38,304 That will be the stronger one. 213 00:10:38,304 --> 00:10:41,173 And you know that bond is going to be in there. 214 00:10:41,173 --> 00:10:43,809 It's going to bond to other hydrogens from other chains 215 00:10:43,809 --> 00:10:45,945 or maybe from itself. 216 00:10:45,945 --> 00:10:49,348 Because remember this is what it looks like. 217 00:10:49,348 --> 00:10:52,018 But even much, much, much, much, much more. 218 00:10:52,018 --> 00:10:54,920 These are macromolecules. 219 00:10:54,920 --> 00:10:56,622 These are macromolecules. 220 00:10:56,622 --> 00:10:58,357 OK. 221 00:10:58,357 --> 00:10:58,891 Yes. 222 00:10:58,891 --> 00:11:04,263 But now I wanted to put these here so I had them. 223 00:11:04,263 --> 00:11:06,298 So I'm going to take one of these. 224 00:11:06,298 --> 00:11:06,999 Which one is it? 225 00:11:06,999 --> 00:11:09,135 It's polystyrene. 226 00:11:09,135 --> 00:11:14,173 It's this one with the very poorly drawn benzene ring. . 227 00:11:14,173 --> 00:11:16,709 And I didn't change anything about it 228 00:11:16,709 --> 00:11:18,744 but look at what it can do. 229 00:11:18,744 --> 00:11:23,482 That's polystyrene in three different forms. 230 00:11:23,482 --> 00:11:26,218 They're all cups but you know these cups well, right? 231 00:11:26,218 --> 00:11:27,453 One's Styrofoam. 232 00:11:27,453 --> 00:11:29,288 One, I don't know what you call that. 233 00:11:29,288 --> 00:11:32,491 But they're all plastic cups but they 234 00:11:32,491 --> 00:11:34,694 have very different properties. 235 00:11:34,694 --> 00:11:37,997 They have the exact same polymer. 236 00:11:37,997 --> 00:11:39,899 Polystyrene. 237 00:11:39,899 --> 00:11:41,133 How do we do that? 238 00:11:41,133 --> 00:11:42,368 That's not the IMFs. 239 00:11:42,368 --> 00:11:45,237 It doesn't even have to have anything 240 00:11:45,237 --> 00:11:46,472 to do with a molecular weight. 241 00:11:46,472 --> 00:11:48,207 It could. 242 00:11:48,207 --> 00:11:50,109 No, it's something else. 243 00:11:50,109 --> 00:11:54,080 And that is the density. 244 00:11:54,080 --> 00:11:57,683 Oh and so now we get to talk about density 245 00:11:57,683 --> 00:12:00,786 and crystallinity. 246 00:12:00,786 --> 00:12:05,825 And I just saved three letters. 247 00:12:05,825 --> 00:12:07,393 Makes me very happy. 248 00:12:07,393 --> 00:12:15,401 Now crystallinity density, memories. 249 00:12:15,401 --> 00:12:16,902 Good memories. 250 00:12:16,902 --> 00:12:21,040 Very good memories are happening right now 251 00:12:21,040 --> 00:12:23,843 from when we talked about amorphous materials 252 00:12:23,843 --> 00:12:27,780 and the glass transition, right? 253 00:12:27,780 --> 00:12:31,517 Because remember, I drew this on the board. 254 00:12:31,517 --> 00:12:32,218 Let's see. 255 00:12:32,218 --> 00:12:36,722 Let's get these to come down. 256 00:12:36,722 --> 00:12:38,790 What that actually looks like-- 257 00:12:42,161 --> 00:12:45,231 each strand in there, remember, goes like this. 258 00:12:45,231 --> 00:12:46,398 And then it's like, oh wait. 259 00:12:46,398 --> 00:12:48,601 I can make a crystal. 260 00:12:48,601 --> 00:12:49,702 And then it's like, nope. 261 00:12:49,702 --> 00:12:50,669 I'm amorphous again. 262 00:12:50,669 --> 00:12:51,804 And then it's like oh wait. 263 00:12:51,804 --> 00:12:52,838 Hold on. 264 00:12:52,838 --> 00:12:54,573 I can crystallize right here. 265 00:12:54,573 --> 00:12:57,576 And then it's amorphous again and it's so fun. 266 00:12:57,576 --> 00:13:00,379 And then you go like this and that. 267 00:13:00,379 --> 00:13:04,984 And that's like one one hundredth of a strand. 268 00:13:04,984 --> 00:13:11,524 And so you know the crystalline region-- 269 00:13:11,524 --> 00:13:12,858 what do I mean by "crystalline?" 270 00:13:12,858 --> 00:13:15,661 Well, the strand is stacking up in some 271 00:13:15,661 --> 00:13:17,997 ordered regular repeating way. 272 00:13:17,997 --> 00:13:19,231 What do I mean by "amorphous?" 273 00:13:19,231 --> 00:13:22,968 Well, I mean spaghetti. 274 00:13:22,968 --> 00:13:25,504 But a polymer, in its own strand, 275 00:13:25,504 --> 00:13:30,376 can crystallize to some extent or to more or less, right? 276 00:13:30,376 --> 00:13:31,877 And that's something we can control. 277 00:13:31,877 --> 00:13:33,846 We can engineer that. 278 00:13:33,846 --> 00:13:36,582 And that's what leads to these differences 279 00:13:36,582 --> 00:13:41,153 because you know that in a crystalline region, 280 00:13:41,153 --> 00:13:46,192 it's going to have a lower volume, right? 281 00:13:46,192 --> 00:13:48,594 It's going to take up less volume. 282 00:13:48,594 --> 00:13:50,963 That's what we did before with glasses. 283 00:13:50,963 --> 00:13:53,833 So let's put that on the board, right? 284 00:13:53,833 --> 00:13:58,571 So you know that from out good old days 285 00:13:58,571 --> 00:14:04,510 that if I melt a material, well, it could form a crystal. 286 00:14:04,510 --> 00:14:05,945 That's temperature. 287 00:14:05,945 --> 00:14:09,215 And this would be volume per mole. 288 00:14:09,215 --> 00:14:11,016 Right, this would be like the melting point. 289 00:14:11,016 --> 00:14:12,084 It might form a crystal. 290 00:14:12,084 --> 00:14:14,787 But with polymers, imagine I've got the spaghetti strands 291 00:14:14,787 --> 00:14:16,822 and now they're miles long. 292 00:14:16,822 --> 00:14:19,491 It's really hard to line it all up. 293 00:14:19,491 --> 00:14:24,496 And so a lot of it will not find that place. 294 00:14:24,496 --> 00:14:27,833 And instead it will be a glass, which 295 00:14:27,833 --> 00:14:32,104 you know is another way of saying so these are now TGs, 296 00:14:32,104 --> 00:14:32,605 right? 297 00:14:32,605 --> 00:14:34,506 TG. 298 00:14:34,506 --> 00:14:37,977 And so you know now all about this, right? 299 00:14:37,977 --> 00:14:40,512 And you know that cooling rate is 300 00:14:40,512 --> 00:14:43,649 one way to change the density. 301 00:14:43,649 --> 00:14:48,454 To change the volume per mole, right? 302 00:14:48,454 --> 00:14:52,024 So the density of the amorphous region 303 00:14:52,024 --> 00:14:56,095 is one parameter that could be glassy in different ways, 304 00:14:56,095 --> 00:14:56,996 like we did before. 305 00:14:56,996 --> 00:15:00,032 And then how much of it you can make into a crystal 306 00:15:00,032 --> 00:15:01,934 is another parameter. 307 00:15:01,934 --> 00:15:04,937 Both of which play into the overall density 308 00:15:04,937 --> 00:15:06,872 and crystallinity, right? 309 00:15:06,872 --> 00:15:09,575 Both of those are extremely important 310 00:15:09,575 --> 00:15:13,012 and those are going to depend on all sorts of parameters related 311 00:15:13,012 --> 00:15:16,015 to the processing and the temperature and so forth. 312 00:15:16,015 --> 00:15:18,817 Those are the things that can lead to changes like this 313 00:15:18,817 --> 00:15:21,553 from the same exact chemistry. 314 00:15:21,553 --> 00:15:29,028 So let's go from polystyrene back to polyethylene. 315 00:15:29,028 --> 00:15:31,730 So if you look at polyethylene-- 316 00:15:31,730 --> 00:15:32,464 so this is a list. 317 00:15:32,464 --> 00:15:34,600 I showed I think a version of this on Monday. 318 00:15:38,270 --> 00:15:39,038 It's hard to read. 319 00:15:39,038 --> 00:15:39,972 Don't worry about it. 320 00:15:39,972 --> 00:15:42,841 These are all monomers. 321 00:15:42,841 --> 00:15:43,842 And this is the polymer. 322 00:15:43,842 --> 00:15:45,110 They didn't put the brackets there 323 00:15:45,110 --> 00:15:46,679 but you can see-- oh maybe they did. 324 00:15:46,679 --> 00:15:47,813 Yeah sort of. 325 00:15:47,813 --> 00:15:51,317 You can see the line, right, coming out. 326 00:15:51,317 --> 00:15:52,952 That's the repeat line. 327 00:15:52,952 --> 00:15:54,520 And this is the monomer. 328 00:15:54,520 --> 00:16:00,159 Notice the double bonds ready for chain polymerization. 329 00:16:00,159 --> 00:16:02,261 And here are the properties and here are the uses. 330 00:16:02,261 --> 00:16:06,432 Look at this-- LDPE and HDPE. 331 00:16:06,432 --> 00:16:08,233 Now LDPE and HDPE. 332 00:16:08,233 --> 00:16:13,138 So that's low density and high density 333 00:16:13,138 --> 00:16:15,040 of the same exact polymer. 334 00:16:15,040 --> 00:16:18,744 So the interactions are the same. 335 00:16:18,744 --> 00:16:20,779 But all we've done is we've changed the density 336 00:16:20,779 --> 00:16:23,849 and the difference between the properties is tremendous. 337 00:16:23,849 --> 00:16:25,417 So when you go to the grocery store 338 00:16:25,417 --> 00:16:29,054 and they give you a plastic bag, which you'll wind up feeding 339 00:16:29,054 --> 00:16:34,159 to fishes in the ocean as you now know, it's very soft. 340 00:16:34,159 --> 00:16:36,095 It's the exact same material that's 341 00:16:36,095 --> 00:16:40,799 in over 30% of all toys on the planet. 342 00:16:40,799 --> 00:16:42,301 It's still polyethylene. 343 00:16:42,301 --> 00:16:45,771 All we've done is mess with that. 344 00:16:45,771 --> 00:16:49,508 We've messed with the density. 345 00:16:49,508 --> 00:16:51,577 So how do you mess with the density? 346 00:16:51,577 --> 00:16:52,511 How? 347 00:16:52,511 --> 00:16:55,647 Well, it turns out with polymers there are 348 00:16:55,647 --> 00:16:57,349 different ways you can do this. 349 00:16:57,349 --> 00:17:00,152 One we just talked about. 350 00:17:00,152 --> 00:17:04,189 So one, let's do a little-- 351 00:17:04,189 --> 00:17:05,424 I'm going to do this. 352 00:17:05,424 --> 00:17:11,030 So one would be let's say processing. 353 00:17:11,030 --> 00:17:12,131 I'll call it processing. 354 00:17:16,234 --> 00:17:17,269 That's right here. 355 00:17:17,269 --> 00:17:19,070 How quickly did I melt it? 356 00:17:19,070 --> 00:17:21,839 What were the conditions that I formed it in? 357 00:17:21,839 --> 00:17:24,777 But see you can imagine changing its ability 358 00:17:24,777 --> 00:17:27,613 to crystallize our pack by just changing something 359 00:17:27,613 --> 00:17:32,885 else about the polymer itself. 360 00:17:32,885 --> 00:17:35,587 And so another way to do that would 361 00:17:35,587 --> 00:17:40,325 be to change the physical structure of the chain. 362 00:17:40,325 --> 00:17:41,794 Physical structure. 363 00:17:45,731 --> 00:17:49,101 And there are two ways that you can do this. 364 00:17:49,101 --> 00:17:50,035 Oh boy, let's see. 365 00:17:50,035 --> 00:17:52,838 How am I going to continue this? 366 00:17:52,838 --> 00:17:56,108 OK let's do this. 367 00:17:56,108 --> 00:17:57,376 Bullet one. 368 00:17:57,376 --> 00:17:59,478 One way is called branching. 369 00:18:03,282 --> 00:18:05,684 And another is called tacticity. 370 00:18:08,687 --> 00:18:09,822 So what are these? 371 00:18:09,822 --> 00:18:14,893 So branching is exactly what it sounds like because the polymer 372 00:18:14,893 --> 00:18:15,828 I showed you here-- 373 00:18:18,464 --> 00:18:20,966 this is a mess and it's showing how it's a spaghetti chain. 374 00:18:20,966 --> 00:18:24,069 But notice that it's just a line everywhere. 375 00:18:24,069 --> 00:18:28,440 We would call this a straight line. 376 00:18:28,440 --> 00:18:31,877 This is a straight line, it just kind of wanders around. 377 00:18:31,877 --> 00:18:32,644 Yeah. 378 00:18:32,644 --> 00:18:33,679 OK. 379 00:18:33,679 --> 00:18:39,952 Because it's in contrast to-- 380 00:18:39,952 --> 00:18:43,522 if it's a straight line, then it's sort of locally straight. 381 00:18:43,522 --> 00:18:44,690 But what if it did this? 382 00:18:47,860 --> 00:18:49,695 And then it's like, well, which way do I go? 383 00:18:53,332 --> 00:18:55,134 You can be the exact same chemistry 384 00:18:55,134 --> 00:18:58,470 but because maybe one of those radicals in the soup while this 385 00:18:58,470 --> 00:19:01,673 was being made came in and got to move a hydrogen around 386 00:19:01,673 --> 00:19:02,774 in just the right way. 387 00:19:02,774 --> 00:19:05,511 It was able to start growing off of the side, right? 388 00:19:05,511 --> 00:19:08,213 That's a branch. 389 00:19:08,213 --> 00:19:13,218 So you can imagine that controlling this branching 390 00:19:13,218 --> 00:19:15,521 would be a really big deal in terms 391 00:19:15,521 --> 00:19:18,891 of the crystallinity right? 392 00:19:18,891 --> 00:19:23,295 So now if you go to pasta. 393 00:19:23,295 --> 00:19:25,898 It always seems to come back to pasta. 394 00:19:25,898 --> 00:19:27,699 Maybe it's just me. 395 00:19:27,699 --> 00:19:28,700 I don't think so though. 396 00:19:28,700 --> 00:19:33,438 I think there's something deep about the pasta in here. 397 00:19:33,438 --> 00:19:37,543 But see, this is a polymer. 398 00:19:37,543 --> 00:19:42,080 But you can imagine now if I make spaghetti 399 00:19:42,080 --> 00:19:45,551 and it looks like the left, it's all packed in. 400 00:19:45,551 --> 00:19:48,654 This is actually an interesting idea for a new pasta brand, 401 00:19:48,654 --> 00:19:49,388 right? 402 00:19:49,388 --> 00:19:53,792 And what if each spaghetti strand looked like that? 403 00:19:53,792 --> 00:19:58,230 And now you take those and you try to make your-- 404 00:19:58,230 --> 00:20:01,767 it would be actually pretty cool but it wouldn't pack as well. 405 00:20:01,767 --> 00:20:04,636 You can feel it with the pasta, right? 406 00:20:04,636 --> 00:20:05,304 You can feel it. 407 00:20:05,304 --> 00:20:07,973 It's going to take up more room in the bowl. 408 00:20:07,973 --> 00:20:11,243 I mean, it's the exact same thing with polymers. 409 00:20:11,243 --> 00:20:15,080 So if you want a polymer to be stronger, 410 00:20:15,080 --> 00:20:18,817 to have maybe more crystallinity or higher density, 411 00:20:18,817 --> 00:20:24,022 HDPE, then you don't want very much branching. 412 00:20:24,022 --> 00:20:25,490 So you better dial that in. 413 00:20:25,490 --> 00:20:26,892 When you synthesize it, you better 414 00:20:26,892 --> 00:20:28,827 think carefully about how you're making 415 00:20:28,827 --> 00:20:34,099 this to prevent those kinds of side chains from growing, OK? 416 00:20:34,099 --> 00:20:36,101 Well, tacticity is another thing. 417 00:20:36,101 --> 00:20:38,737 And that's not the same as branching. 418 00:20:38,737 --> 00:20:43,742 Tacticity is remember-- ah! 419 00:20:43,742 --> 00:20:45,477 Where did it go? 420 00:20:45,477 --> 00:20:46,245 I had it up there. 421 00:20:49,348 --> 00:20:52,084 There's the benzene ring. 422 00:20:52,084 --> 00:20:56,588 Are they all on the same side or are they on alternating sides? 423 00:20:56,588 --> 00:20:58,724 How did this thing go? 424 00:20:58,724 --> 00:21:02,928 Did two go on the same side and then one and is it random? 425 00:21:02,928 --> 00:21:06,965 I mean, you can imagine that if I have a way 426 00:21:06,965 --> 00:21:10,202 to break the symmetry of the chain like this, 427 00:21:10,202 --> 00:21:15,641 then you can imagine that if I break it a lot so I'm atactic, 428 00:21:15,641 --> 00:21:19,544 then that's going to be harder to line up as well. 429 00:21:19,544 --> 00:21:22,247 That's going to make it harder for the spaghetti 430 00:21:22,247 --> 00:21:26,051 to kind of fall right next to each other. 431 00:21:26,051 --> 00:21:28,520 But even if this spaghetti had little 432 00:21:28,520 --> 00:21:31,390 you can call these little, mini benzine branches. 433 00:21:31,390 --> 00:21:33,125 It's not branching. 434 00:21:33,125 --> 00:21:34,693 But you can call them little branches. 435 00:21:34,693 --> 00:21:36,194 But if they're all on the same side, 436 00:21:36,194 --> 00:21:40,732 then you can imagine they can still stack really well. 437 00:21:40,732 --> 00:21:43,302 So when you have side groups like this, 438 00:21:43,302 --> 00:21:46,505 this is another aspect of engineering the polymer 439 00:21:46,505 --> 00:21:48,640 that you got to think very carefully about. 440 00:21:48,640 --> 00:21:52,277 Because if it's isotactic or syndiotactic, 441 00:21:52,277 --> 00:21:53,945 then it's going to be-- 442 00:21:53,945 --> 00:21:56,014 so that's the physical structure of the chain. 443 00:21:56,014 --> 00:21:58,884 --then it's going to be able to pack more easily 444 00:21:58,884 --> 00:22:01,953 and it's going to be stronger, right? 445 00:22:01,953 --> 00:22:04,790 if it's atactic and we're talking about like a hundred 446 00:22:04,790 --> 00:22:06,258 degrees. 447 00:22:06,258 --> 00:22:11,963 Same polymer, same chemical unit make the molecular weights 448 00:22:11,963 --> 00:22:12,464 the same. 449 00:22:12,464 --> 00:22:16,635 All you've changed is whether there's symmetry or not. 450 00:22:16,635 --> 00:22:19,871 And you can change the melting point by a hundred degrees. 451 00:22:19,871 --> 00:22:21,039 Right, of the same material. 452 00:22:21,039 --> 00:22:23,642 So that's another way that we engineer 453 00:22:23,642 --> 00:22:27,512 the properties of polymers, OK? 454 00:22:27,512 --> 00:22:32,084 Same polymer garbage bags to Tupperware. 455 00:22:32,084 --> 00:22:33,585 Same chemistry. 456 00:22:33,585 --> 00:22:37,122 Isotactic to atactic. 457 00:22:37,122 --> 00:22:42,394 OK, now we get to something really fun, 458 00:22:42,394 --> 00:22:44,329 which is cross-links. 459 00:22:44,329 --> 00:22:49,368 And this is number five, OK? 460 00:22:49,368 --> 00:22:54,072 So we're talking about ways to control polymers. 461 00:22:54,072 --> 00:22:57,642 Cross-linking is one of the single most important ways 462 00:22:57,642 --> 00:23:00,746 to engineer the properties of polymers. 463 00:23:00,746 --> 00:23:02,714 And it basically is exactly what it 464 00:23:02,714 --> 00:23:06,618 says-- you're linking two different polymer 465 00:23:06,618 --> 00:23:08,887 chains together across. 466 00:23:08,887 --> 00:23:12,724 Cross linking, OK? 467 00:23:12,724 --> 00:23:14,826 Now, it turns out the one-- remember, 468 00:23:14,826 --> 00:23:17,829 I showed you the video of the nylon. 469 00:23:17,829 --> 00:23:19,164 It's called the nylon rope pole. 470 00:23:19,164 --> 00:23:20,866 If you want to see that again, Google it. 471 00:23:20,866 --> 00:23:21,933 It's so cool. 472 00:23:21,933 --> 00:23:24,603 You have two liquids with the two-- 473 00:23:24,603 --> 00:23:27,472 it's a condensation polymerization example. 474 00:23:27,472 --> 00:23:30,409 Two liquids with two different monomers and you 475 00:23:30,409 --> 00:23:32,077 pull a solid out of the interface. 476 00:23:32,077 --> 00:23:34,279 And that's this. 477 00:23:34,279 --> 00:23:35,747 That's exactly this. 478 00:23:35,747 --> 00:23:37,449 And those are the molecules, these 479 00:23:37,449 --> 00:23:39,351 are the monomers that were in the liquid. 480 00:23:39,351 --> 00:23:42,053 And there is nylon 6.6. 481 00:23:42,053 --> 00:23:43,989 Remember, it's because you have a chain there. 482 00:23:43,989 --> 00:23:44,723 And this is what you get. 483 00:23:44,723 --> 00:23:45,924 And it keeps repeating. 484 00:23:45,924 --> 00:23:48,627 And there's your polymer. 485 00:23:48,627 --> 00:23:52,464 The reason I'm saying that nylon actually cross-links 486 00:23:52,464 --> 00:23:54,132 is because it's something very important. 487 00:23:54,132 --> 00:23:58,837 It's not just that these have different interactions 488 00:23:58,837 --> 00:23:59,738 with each other. 489 00:23:59,738 --> 00:24:00,372 No. 490 00:24:00,372 --> 00:24:02,174 It's something a little bit more than that. 491 00:24:02,174 --> 00:24:05,644 So if I look at this and say, OK, here's a carbon 492 00:24:05,644 --> 00:24:07,212 and a nitrogen. I'm going to just try 493 00:24:07,212 --> 00:24:09,915 to draw a part of this. 494 00:24:09,915 --> 00:24:13,251 Carbon, nitrogen, there's a hydrogen, 495 00:24:13,251 --> 00:24:16,388 and oh boy, here we go. 496 00:24:16,388 --> 00:24:18,490 Here's an oxygen. OK. 497 00:24:18,490 --> 00:24:19,257 And it goes on. 498 00:24:19,257 --> 00:24:20,992 Oh, I'm not going to draw what's in there. 499 00:24:20,992 --> 00:24:22,127 It's 6.6. 500 00:24:22,127 --> 00:24:24,496 And then comes another. 501 00:24:24,496 --> 00:24:27,999 OK, nitrogen, hydrogen, and then it 502 00:24:27,999 --> 00:24:33,305 goes up to a carbon, oxygen, and then ah, 6.6. 503 00:24:33,305 --> 00:24:34,973 Or whatever it is, right? 504 00:24:34,973 --> 00:24:39,144 And then it goes to a carbon and oxygen. 505 00:24:39,144 --> 00:24:40,545 This should have been like that. 506 00:24:40,545 --> 00:24:48,753 And this goes to nitrogen and so on and so on. 507 00:24:48,753 --> 00:24:51,823 Oh remember, this is called the amide bond. 508 00:24:51,823 --> 00:24:52,757 That's cool, right? 509 00:24:52,757 --> 00:24:53,925 That's the bond that formed. 510 00:24:57,295 --> 00:24:58,196 That's the amide bond. 511 00:24:58,196 --> 00:25:01,399 That's the bond that formed this polyamide, right? 512 00:25:01,399 --> 00:25:03,668 We did this on Monday. 513 00:25:03,668 --> 00:25:06,571 But look at what happened. 514 00:25:06,571 --> 00:25:09,541 So if I have another one of these 515 00:25:09,541 --> 00:25:11,209 that comes up beneath it-- and I'm not 516 00:25:11,209 --> 00:25:12,444 going to draw the whole thing. 517 00:25:12,444 --> 00:25:16,214 I'm just going to draw that part that has the amide bond. 518 00:25:16,214 --> 00:25:21,219 You can see that if I have an oxygen here with the carbon 519 00:25:21,219 --> 00:25:27,092 and let's do oh boy, nitrogen, and like that OK? 520 00:25:27,092 --> 00:25:31,129 And so on, then if this lines up-- 521 00:25:31,129 --> 00:25:33,932 but it can line up. 522 00:25:33,932 --> 00:25:37,369 And then over here you've got a hydrogen. 523 00:25:37,369 --> 00:25:39,738 And so you can get this kind of really nice line 524 00:25:39,738 --> 00:25:46,978 up to form a hydrogen bond between these chains. 525 00:25:46,978 --> 00:25:47,846 You can. 526 00:25:47,846 --> 00:25:49,381 It can happen. 527 00:25:49,381 --> 00:25:52,384 And that can add, as we know, that can add strength 528 00:25:52,384 --> 00:25:53,952 to the material. 529 00:25:53,952 --> 00:25:57,689 So that's a cross link because it's a very specific linkage 530 00:25:57,689 --> 00:26:02,360 between these strands, right? 531 00:26:02,360 --> 00:26:04,596 It's not just sort of an average over different types 532 00:26:04,596 --> 00:26:06,164 of bonding. 533 00:26:06,164 --> 00:26:10,368 It's the specific link that's in the chain. 534 00:26:10,368 --> 00:26:14,806 Well, it turns out that that just seemed to happen. 535 00:26:14,806 --> 00:26:17,809 Because of the chemistry that was there it can happen. 536 00:26:17,809 --> 00:26:21,246 But look, we can also make it happen. 537 00:26:21,246 --> 00:26:25,250 We can engineer polymers and we do 538 00:26:25,250 --> 00:26:30,055 to have all sorts of cross-linking chemistry. 539 00:26:30,055 --> 00:26:33,191 And it's really one of the most single important ways 540 00:26:33,191 --> 00:26:35,727 to control the properties of polymers, right? 541 00:26:35,727 --> 00:26:38,129 So I want to talk about that a little bit. 542 00:26:38,129 --> 00:26:40,599 And I'm going to do it with maybe 543 00:26:40,599 --> 00:26:47,339 the most well-used, at least, example of what 544 00:26:47,339 --> 00:26:48,607 a cross-linker does. 545 00:26:48,607 --> 00:26:50,909 We're going to talk about rubber. 546 00:26:50,909 --> 00:26:56,114 Now, rubber is a natural polymer. 547 00:26:56,114 --> 00:26:58,516 You can get it out of trees, right? 548 00:26:58,516 --> 00:27:00,452 I think maybe I have a picture. 549 00:27:00,452 --> 00:27:01,586 Yeah, I do. 550 00:27:01,586 --> 00:27:03,388 Oh, poor Goodyear. 551 00:27:03,388 --> 00:27:07,292 But we'll talk about him in a minute. 552 00:27:07,292 --> 00:27:09,694 There's rubber coming right out of a tree. 553 00:27:09,694 --> 00:27:13,865 And when we see rubber in this class we think of isoprene. 554 00:27:13,865 --> 00:27:18,670 And isoprene is this molecule, OK? 555 00:27:18,670 --> 00:27:20,205 So here's isoprene. 556 00:27:20,205 --> 00:27:26,244 There's a CH3 and then there's a double bond 557 00:27:26,244 --> 00:27:30,548 to a CH2 and another double bond to another CH2. 558 00:27:33,785 --> 00:27:44,396 Now, the polymer that is coming from this rubber tree is what? 559 00:27:44,396 --> 00:27:46,097 It's polyisoprene, OK? 560 00:27:46,097 --> 00:27:50,502 So I've got two double bonds. 561 00:27:50,502 --> 00:27:52,303 We're going to talk about that in a second. 562 00:27:52,303 --> 00:27:55,740 But I only need one to polymerize this. 563 00:27:55,740 --> 00:27:58,410 I only need one, right? 564 00:27:58,410 --> 00:28:04,215 And so if I think about this, I can make a polymer out of it 565 00:28:04,215 --> 00:28:06,351 and the polymer would look like this. 566 00:28:06,351 --> 00:28:08,319 This would be polyisoprene, OK? 567 00:28:08,319 --> 00:28:11,690 So H, I'll draw the Hs in just so we keep track. 568 00:28:11,690 --> 00:28:15,126 Here's another C. And this has my-- 569 00:28:15,126 --> 00:28:15,694 oh boy. 570 00:28:15,694 --> 00:28:18,963 I said I'd draw them and now I have to. 571 00:28:18,963 --> 00:28:22,467 Once you commit, all right, there it is. 572 00:28:22,467 --> 00:28:23,435 H, right? 573 00:28:23,435 --> 00:28:26,237 And then you've got the double bond in there. 574 00:28:26,237 --> 00:28:28,173 And an H there. 575 00:28:28,173 --> 00:28:33,344 And then you've got this CH2 unit there. 576 00:28:33,344 --> 00:28:37,582 Oh, and that is polyisoprene. 577 00:28:37,582 --> 00:28:40,418 It's not-- it looks complicated. 578 00:28:40,418 --> 00:28:44,155 It's got a CH3 unit and some CH2 units. 579 00:28:44,155 --> 00:28:46,224 But here's the key-- 580 00:28:46,224 --> 00:28:49,094 it's got left over, a double bond. 581 00:28:49,094 --> 00:28:51,529 All right, that's the key. 582 00:28:51,529 --> 00:28:58,670 Now, you see, that double bond, I can do more chemistry with. 583 00:28:58,670 --> 00:29:01,406 I can do cross-link chemistry with. 584 00:29:05,376 --> 00:29:08,980 So I started with two and I'm left with one 585 00:29:08,980 --> 00:29:11,683 because I needed one to make the 100,000 586 00:29:11,683 --> 00:29:14,619 long chains of these things, right so this is isoprene. 587 00:29:17,222 --> 00:29:19,424 And this is polyisoprene. 588 00:29:19,424 --> 00:29:21,693 Yeah, but the thing is that polyisoprene is not 589 00:29:21,693 --> 00:29:22,227 very useful. 590 00:29:24,896 --> 00:29:26,197 The Mayans used it. 591 00:29:26,197 --> 00:29:29,868 People tried to use polyisoprene directly 592 00:29:29,868 --> 00:29:32,737 from nature for centuries. 593 00:29:32,737 --> 00:29:36,407 The Mayans would because it's soft, you know? 594 00:29:36,407 --> 00:29:37,976 It's like a mold. 595 00:29:37,976 --> 00:29:40,044 It's this really cool mold. 596 00:29:40,044 --> 00:29:42,814 And probably, at some point, somebody stepped in it. 597 00:29:42,814 --> 00:29:45,817 And they're like, look, my footprint. 598 00:29:45,817 --> 00:29:46,885 And it stayed. 599 00:29:46,885 --> 00:29:50,054 And so then, what they did is they made boots out of it. 600 00:29:50,054 --> 00:29:51,623 And so they would put their whole foot 601 00:29:51,623 --> 00:29:54,759 inside of bucket of isoprene. 602 00:29:54,759 --> 00:29:58,062 and then it would sort of mold really well, right? 603 00:29:58,062 --> 00:29:59,631 And they'd make shoes that way. 604 00:29:59,631 --> 00:30:03,001 The problem is people tried to do this much later too. 605 00:30:03,001 --> 00:30:06,871 The problem with making a boot out of natural rubber 606 00:30:06,871 --> 00:30:09,440 is that the natural rubber doesn't hold together 607 00:30:09,440 --> 00:30:10,241 very well. 608 00:30:10,241 --> 00:30:13,378 So like seriously, on a hot day, the boot 609 00:30:13,378 --> 00:30:17,315 melts, which is not what you want. 610 00:30:17,315 --> 00:30:19,784 People made clothing out of it too. 611 00:30:19,784 --> 00:30:24,489 Jackets were made with polyisoprene inside 612 00:30:24,489 --> 00:30:25,456 of two fibers. 613 00:30:25,456 --> 00:30:28,993 But then it would melt and get like sticky, like, on a summer 614 00:30:28,993 --> 00:30:32,497 day or if it rained out. 615 00:30:32,497 --> 00:30:34,632 So you had some issues with rubber. 616 00:30:34,632 --> 00:30:37,502 But people really wanted to make something out of this stuff. 617 00:30:37,502 --> 00:30:39,103 It seemed really neat. 618 00:30:39,103 --> 00:30:44,242 It could kind of be strong but then very moldable. 619 00:30:44,242 --> 00:30:47,378 And so along comes Charles Goodyear. 620 00:30:47,378 --> 00:30:54,953 And he accidentally left rubber on his stove with sulfur. 621 00:30:54,953 --> 00:30:56,921 And literally, the way the story goes 622 00:30:56,921 --> 00:31:01,593 is his house started smelling like burnt rubber, which 623 00:31:01,593 --> 00:31:06,798 was probably the first time ever that smell was made. 624 00:31:06,798 --> 00:31:11,569 And what he found was that this rubber was completely different 625 00:31:11,569 --> 00:31:14,239 when it was left on the stove with sulfur. 626 00:31:14,239 --> 00:31:17,342 It all comes back to the double bond. 627 00:31:17,342 --> 00:31:19,143 It all comes back to the double bond. 628 00:31:19,143 --> 00:31:19,811 Why? 629 00:31:19,811 --> 00:31:22,747 Because now I've got a place where 630 00:31:22,747 --> 00:31:25,149 I can add a covalent link. 631 00:31:25,149 --> 00:31:28,586 And that's exactly what sulfur does, all right? 632 00:31:28,586 --> 00:31:34,659 So if I take this-- 633 00:31:34,659 --> 00:31:35,727 I'll try to draw it again. 634 00:31:35,727 --> 00:31:39,297 I'm not going to draw the Hs this time. 635 00:31:39,297 --> 00:31:43,801 So I've got this and here's that special double bond, right? 636 00:31:43,801 --> 00:31:48,506 And then I've got this and this. 637 00:31:48,506 --> 00:31:50,575 OK, that's natural rubber. 638 00:31:50,575 --> 00:31:53,177 But now see, with a double bond, I 639 00:31:53,177 --> 00:31:54,812 can-- remember, what are these? 640 00:31:54,812 --> 00:31:57,982 Well, these are four electrons? 641 00:31:57,982 --> 00:32:00,919 But if sulfur comes along, I might 642 00:32:00,919 --> 00:32:07,292 be able to take two of them and move them like this. 643 00:32:07,292 --> 00:32:09,627 If sulfur comes and offers something 644 00:32:09,627 --> 00:32:12,263 that the carbon wants. 645 00:32:12,263 --> 00:32:13,965 Hey, come and bond with me. 646 00:32:13,965 --> 00:32:15,733 It'll be more fun. 647 00:32:15,733 --> 00:32:18,803 Maybe we'll go to a lower energy state, right? 648 00:32:18,803 --> 00:32:20,571 And then the carbon's like, you know what? 649 00:32:20,571 --> 00:32:21,940 I get that. 650 00:32:21,940 --> 00:32:23,341 That sounds pretty cool. 651 00:32:23,341 --> 00:32:25,510 I'm going to give an electron from this double bond. 652 00:32:25,510 --> 00:32:28,746 I get to stay connected here with a single bond, right? 653 00:32:28,746 --> 00:32:32,283 And then I can be here and here and the sulfur 654 00:32:32,283 --> 00:32:36,087 can provide that other electron and form a nice single bond, 655 00:32:36,087 --> 00:32:36,587 right? 656 00:32:40,091 --> 00:32:41,693 Because there's that double bond, 657 00:32:41,693 --> 00:32:46,431 you can do the chemistry to create covalent strong bonds 658 00:32:46,431 --> 00:32:46,965 to sulfur. 659 00:32:46,965 --> 00:32:50,068 And then you get like, another sulfur and another sulfur 660 00:32:50,068 --> 00:32:56,341 and so on and these can be maybe 2, 3, 4 sulfur units long, 661 00:32:56,341 --> 00:32:59,010 depending on how you cook it. 662 00:32:59,010 --> 00:33:00,211 And then this will come. 663 00:33:00,211 --> 00:33:03,047 And this comes down and finds another one, 664 00:33:03,047 --> 00:33:05,083 finds another strand. 665 00:33:05,083 --> 00:33:07,018 And it finds the double bond that's 666 00:33:07,018 --> 00:33:10,188 in another polyisoprene strand. 667 00:33:10,188 --> 00:33:15,526 That's called cross-- that's well, accidental. 668 00:33:15,526 --> 00:33:16,427 He was a tinkerer. 669 00:33:16,427 --> 00:33:17,862 And he played with lots of things. 670 00:33:17,862 --> 00:33:20,965 And his greatest discovery was completely by accident. 671 00:33:20,965 --> 00:33:22,533 And he never made any money off of it. 672 00:33:22,533 --> 00:33:24,569 Goodyear Tire, this is Charles Goodyear. 673 00:33:24,569 --> 00:33:26,537 Goodyear tires, this is the company. 674 00:33:26,537 --> 00:33:31,075 It was called the Goodyear Vulcanite Company. 675 00:33:31,075 --> 00:33:33,478 Goodyear called it vulcanization. 676 00:33:33,478 --> 00:33:38,649 And the reason is Vulcan is the Roman god for fire. 677 00:33:38,649 --> 00:33:42,020 And he accidentally left it on the fire. 678 00:33:42,020 --> 00:33:46,190 And so that's called vulcanization because of that. 679 00:33:46,190 --> 00:33:48,793 So this used to be the Goodyear Vulcanite Company. 680 00:33:48,793 --> 00:33:51,963 It's now Goodyear Tires, right? 681 00:33:51,963 --> 00:33:52,697 And what happened? 682 00:33:52,697 --> 00:33:54,832 What happened is this low sulfur bonds 683 00:33:54,832 --> 00:33:58,102 that came about accidentally, the covalent bonds, 684 00:33:58,102 --> 00:34:00,338 held that material together. 685 00:34:00,338 --> 00:34:06,844 And it gave it a controllable flexibility and hardness right? 686 00:34:06,844 --> 00:34:09,746 That's what they were after. 687 00:34:09,746 --> 00:34:13,117 That's what they were after-- flexibility and hardness. 688 00:34:13,117 --> 00:34:20,358 And so what is actually happening in this? 689 00:34:20,358 --> 00:34:23,327 Well, what's happening is you're adding 690 00:34:23,327 --> 00:34:25,496 these covalent bonds between these very, very long 691 00:34:25,496 --> 00:34:27,098 strands, right? 692 00:34:27,098 --> 00:34:34,338 And so you can imagine that if you pull on that material now, 693 00:34:34,338 --> 00:34:38,342 well, the long strands can detangle. 694 00:34:38,342 --> 00:34:43,147 So in the rubber tree, these long, long, long, long strands, 695 00:34:43,147 --> 00:34:44,114 they're all wrapped up. 696 00:34:44,114 --> 00:34:45,349 It's a spaghetti bowl. 697 00:34:45,349 --> 00:34:47,819 And I start pulling. 698 00:34:47,819 --> 00:34:50,088 And they don't break but they start to kind 699 00:34:50,088 --> 00:34:54,025 of slide and detangle, right? 700 00:34:54,025 --> 00:34:56,159 And eventually, you just break it all apart. 701 00:34:56,159 --> 00:35:00,531 But now, I've got a lock on it because each little piece 702 00:35:00,531 --> 00:35:04,135 of spaghetti is tied to another one somewhere. 703 00:35:04,135 --> 00:35:09,373 So I can untangle them only to a point. 704 00:35:09,373 --> 00:35:13,311 And then these linkages kick in. 705 00:35:13,311 --> 00:35:14,612 And they're like, no I'm sorry. 706 00:35:14,612 --> 00:35:17,715 You can't stretch anymore, right? 707 00:35:17,715 --> 00:35:21,519 That's what the crossing does, it gives it that strength. 708 00:35:21,519 --> 00:35:25,056 And so you can imagine, if I had more and more cross-linking, 709 00:35:25,056 --> 00:35:26,991 I might have a stronger material, right? 710 00:35:26,991 --> 00:35:29,694 The stress strain curve could go like that. 711 00:35:29,694 --> 00:35:30,761 Not much cross-linking. 712 00:35:30,761 --> 00:35:33,598 A lot of cross-linking, right? 713 00:35:33,598 --> 00:35:35,099 Because more and more of it is going 714 00:35:35,099 --> 00:35:41,539 to be dictated by these linkages here between the strands. 715 00:35:41,539 --> 00:35:43,307 And then there's something really cool 716 00:35:43,307 --> 00:35:45,376 that can happen with cross-linking because this is 717 00:35:45,376 --> 00:35:47,645 an example of a covalent bond. 718 00:35:47,645 --> 00:35:49,247 This is covalent. 719 00:35:53,284 --> 00:35:55,386 But cross-linking can be-- 720 00:35:59,123 --> 00:36:05,897 let's see, it can be covalent, it can be hydrogen. 721 00:36:05,897 --> 00:36:09,500 We already saw that there's sort of natural cross-linking 722 00:36:09,500 --> 00:36:11,335 in nylon because of that hydrogen bond. 723 00:36:11,335 --> 00:36:14,539 You can introduce hydrogen bonds as well. 724 00:36:14,539 --> 00:36:16,340 You can be purposeful about it. 725 00:36:16,340 --> 00:36:17,675 By the way, you can have ionic-- 726 00:36:22,613 --> 00:36:27,151 you can put any kind of bonding that you can dial in, 727 00:36:27,151 --> 00:36:29,720 and there are different ways to do that, 728 00:36:29,720 --> 00:36:32,390 can become a cross-link opportunity, right? 729 00:36:32,390 --> 00:36:37,361 So you can just imagine how much this opened up polymer design. 730 00:36:37,361 --> 00:36:41,432 How much this cross-linking opens up polymer design. 731 00:36:41,432 --> 00:36:47,104 Now, there's a downside as well. 732 00:36:47,104 --> 00:36:49,540 But first, let's say positive. 733 00:36:49,540 --> 00:36:51,209 Let's go to your goody bag. 734 00:36:51,209 --> 00:36:53,911 This is the chemistry inside your goody bag, right? 735 00:36:53,911 --> 00:36:58,316 Now, this is a very purposeful. 736 00:36:58,316 --> 00:36:59,984 Right, these are the Borax cross-linkers 737 00:36:59,984 --> 00:37:02,587 you've got that in your bag and you add it in. 738 00:37:02,587 --> 00:37:05,489 And what I want you to do is get an actual feeling 739 00:37:05,489 --> 00:37:07,858 for the cross-linkers, right? 740 00:37:07,858 --> 00:37:09,794 And so you can add more and you'll 741 00:37:09,794 --> 00:37:11,796 feel the material change because you're 742 00:37:11,796 --> 00:37:12,964 linking more and more of it. 743 00:37:12,964 --> 00:37:15,833 But see, in this case, it's a hydrogen bond. 744 00:37:15,833 --> 00:37:17,201 You see that? 745 00:37:17,201 --> 00:37:18,836 See, those are the Borax cross-linkers. 746 00:37:18,836 --> 00:37:23,574 Those groups keep their OHs and then those OHs, 747 00:37:23,574 --> 00:37:25,276 there's the polyvinyl alcohol. 748 00:37:25,276 --> 00:37:28,646 I drew that on the board somewhere before and those 749 00:37:28,646 --> 00:37:34,285 OHs can now bond to the hydrogen or the OH and the PBA 750 00:37:34,285 --> 00:37:36,320 and that can form a cross-link. 751 00:37:36,320 --> 00:37:39,156 But there's something different about this one 752 00:37:39,156 --> 00:37:40,992 and it's so cool. 753 00:37:40,992 --> 00:37:45,363 And that's why slime is so cool and Silly Putty, right? 754 00:37:45,363 --> 00:37:51,168 Because now there is a timescale involved. 755 00:37:51,168 --> 00:37:52,436 There's a timescale involved. 756 00:37:52,436 --> 00:37:58,209 So now you can pull it very slowly 757 00:37:58,209 --> 00:38:00,911 and the hydrogen bonds that are cross-linking 758 00:38:00,911 --> 00:38:05,383 haven't have time to change, to move, to reform. 759 00:38:05,383 --> 00:38:10,888 Yeah, they break and then they reform, right? 760 00:38:10,888 --> 00:38:13,791 But if you pull it really fast, those hydrogen bonds 761 00:38:13,791 --> 00:38:15,192 don't have time, right? 762 00:38:15,192 --> 00:38:18,262 And I'll show you a picture in a second. 763 00:38:18,262 --> 00:38:22,366 And so what can happen now is the mechanical strength 764 00:38:22,366 --> 00:38:24,035 of this material, the brittleness of it. 765 00:38:24,035 --> 00:38:25,936 The properties of it can literally just 766 00:38:25,936 --> 00:38:28,139 depend on how quickly you pull it. 767 00:38:28,139 --> 00:38:31,709 Because you are up against the speed of hydrogen bond 768 00:38:31,709 --> 00:38:33,444 forming and reforming. 769 00:38:33,444 --> 00:38:36,447 And that is incredibly cool. 770 00:38:36,447 --> 00:38:39,817 You are literally feeling the formation, the destruction 771 00:38:39,817 --> 00:38:42,586 and formation of hydrogen bonds. 772 00:38:42,586 --> 00:38:43,954 But there's something else that's 773 00:38:43,954 --> 00:38:46,624 going on in this that's also so cool. 774 00:38:46,624 --> 00:38:53,197 This material, slime, that polymer is below its glass 775 00:38:53,197 --> 00:38:55,232 transition temperature. 776 00:38:55,232 --> 00:38:56,867 Sorry, above. 777 00:38:56,867 --> 00:38:58,869 I meant above. 778 00:38:58,869 --> 00:39:00,037 What does that mean? 779 00:39:00,037 --> 00:39:03,074 It means that without the cross-linkers, 780 00:39:03,074 --> 00:39:05,076 that polymer is a viscous liquid. 781 00:39:08,646 --> 00:39:10,581 We drew it somewhere. 782 00:39:10,581 --> 00:39:11,549 I erased it? 783 00:39:11,549 --> 00:39:14,085 I erased it. 784 00:39:14,085 --> 00:39:17,154 If you're below the ground the glass transition temperature, 785 00:39:17,154 --> 00:39:21,158 you're basically a solid, like a brittle solid. 786 00:39:21,158 --> 00:39:25,796 Well, like I drew, polymers are amorphous, maybe 787 00:39:25,796 --> 00:39:29,033 with some crystalinity but a lot of it's going to be amorphous. 788 00:39:29,033 --> 00:39:31,135 Maybe all of it. 789 00:39:31,135 --> 00:39:34,705 And so it's going to have a glass transition temperature 790 00:39:34,705 --> 00:39:37,708 and if you're above that, it's this viscous liquid. 791 00:39:37,708 --> 00:39:41,679 It's this like super cooled viscous liquid. 792 00:39:41,679 --> 00:39:43,381 But what I've done with the cross-linkers 793 00:39:43,381 --> 00:39:44,949 is I've said hold on. 794 00:39:44,949 --> 00:39:47,084 You can't go away. 795 00:39:47,084 --> 00:39:51,956 So I'm literally making a solid out of liquid. 796 00:39:51,956 --> 00:39:54,759 We say yeah, just put like, water in the freezer. 797 00:39:54,759 --> 00:39:59,296 No, that's not-- the phase of that polymer is liquid. 798 00:39:59,296 --> 00:40:01,465 It is a viscous liquid. 799 00:40:01,465 --> 00:40:05,102 But it is not allowed to flow or if it does, 800 00:40:05,102 --> 00:40:06,737 it maybe can change its shape a little. 801 00:40:06,737 --> 00:40:09,206 Bit it can't just flow all over the place because 802 00:40:09,206 --> 00:40:11,475 of those cross-links, right? 803 00:40:11,475 --> 00:40:13,277 And that's an elastomer. 804 00:40:13,277 --> 00:40:19,583 And if you do it just right, you get a viscal material. 805 00:40:19,583 --> 00:40:20,684 And what do I mean by that? 806 00:40:20,684 --> 00:40:22,920 Well, all these things have to line up, right? 807 00:40:22,920 --> 00:40:25,956 The thing has to want to be a liquid so it's 808 00:40:25,956 --> 00:40:28,225 got to have the right glass transition temperature. 809 00:40:28,225 --> 00:40:30,494 But the cross-linkers are sort of holding it in place. 810 00:40:30,494 --> 00:40:32,997 And the cross-linker chemistry has to be the right strength. 811 00:40:32,997 --> 00:40:36,133 It can't be too strong, can't be too weak, right? 812 00:40:36,133 --> 00:40:40,337 And again, by accident, you might make cross-link silicone. 813 00:40:40,337 --> 00:40:42,573 Oh there's the goody bag, which is Silly Putty. 814 00:40:42,573 --> 00:40:45,142 And that's why I wanted to give that to you because this hits 815 00:40:45,142 --> 00:40:46,243 all the right combinations. 816 00:40:46,243 --> 00:40:51,315 In this case, the backbone is made out of silicon, right? 817 00:40:51,315 --> 00:40:52,583 Not carbon. 818 00:40:52,583 --> 00:40:53,584 But it's the same thing. 819 00:40:53,584 --> 00:40:56,153 It's the cross-linking that does this. 820 00:40:56,153 --> 00:40:58,522 So there's an example of Silly Putty, which you all 821 00:40:58,522 --> 00:40:59,390 have the goody bag. 822 00:40:59,390 --> 00:41:05,396 And if you stretch it slowly, those hydrogen bonds-- 823 00:41:05,396 --> 00:41:09,200 the strands can kind of pull away. 824 00:41:09,200 --> 00:41:13,337 And those hydrogen bonds have time to slowly form and reform. 825 00:41:13,337 --> 00:41:15,706 That's why it kind of feels the same. 826 00:41:15,706 --> 00:41:19,410 Nice, elastic right? 827 00:41:19,410 --> 00:41:21,412 Depending on the material, on the backbone, 828 00:41:21,412 --> 00:41:23,113 it might go back as well. 829 00:41:23,113 --> 00:41:24,648 In Silly Putty, it kind of just keeps 830 00:41:24,648 --> 00:41:26,884 on-- those strands just keep on sliding 831 00:41:26,884 --> 00:41:28,552 and the hydrogen bonds keep reforming. 832 00:41:28,552 --> 00:41:31,589 But if instead I pull it fast, the hydrogen bonds 833 00:41:31,589 --> 00:41:32,990 don't have time to react. 834 00:41:32,990 --> 00:41:36,227 That's why, if you pull it really fast-- 835 00:41:36,227 --> 00:41:38,195 like OK, don't do this. 836 00:41:38,195 --> 00:41:40,498 Somebody shot a bullet through it. 837 00:41:40,498 --> 00:41:41,532 It breaks like glass. 838 00:41:44,869 --> 00:41:47,371 And you can feel it in your hands, just pull really fast. 839 00:41:47,371 --> 00:41:51,008 The hydrogen bonds don't have time to reform, right? 840 00:41:51,008 --> 00:41:52,810 And so what you're left with is you're just 841 00:41:52,810 --> 00:41:55,513 breaking apart this material. 842 00:41:55,513 --> 00:41:59,917 Now, how elastic it feels, right, how viscous it feels. 843 00:41:59,917 --> 00:42:01,285 All these things, these are going 844 00:42:01,285 --> 00:42:03,888 to be determined by the glass transition 845 00:42:03,888 --> 00:42:07,358 temperature of the temperature of the room, right? 846 00:42:07,358 --> 00:42:10,327 And so that's going to depend on whether you actually 847 00:42:10,327 --> 00:42:12,096 have a very vicious liquid. 848 00:42:12,096 --> 00:42:13,464 Or maybe you've got a solid, then 849 00:42:13,464 --> 00:42:14,698 it's going to be more brittle. 850 00:42:14,698 --> 00:42:17,568 All of these things are now at your disposal 851 00:42:17,568 --> 00:42:19,770 to tune the properties of the polymer, right? 852 00:42:19,770 --> 00:42:20,971 They're all at your disposal. 853 00:42:20,971 --> 00:42:25,409 And it's incredible to think about, for me at least, 854 00:42:25,409 --> 00:42:28,979 holding together a liquid in place like that. 855 00:42:28,979 --> 00:42:33,284 And that's leads to viscoelasticity. 856 00:42:33,284 --> 00:42:35,853 Viscoelasticity, right? 857 00:42:35,853 --> 00:42:39,523 That's just viscous liquid in elastic, right? 858 00:42:39,523 --> 00:42:40,724 OK. 859 00:42:40,724 --> 00:42:45,162 Now, there's a couple more things 860 00:42:45,162 --> 00:42:46,397 that we have to talk about. 861 00:42:46,397 --> 00:42:47,531 And on Friday. 862 00:42:47,531 --> 00:42:49,833 What I want to do Friday is I want to come back 863 00:42:49,833 --> 00:42:51,869 a little bit to some of the ways that we're 864 00:42:51,869 --> 00:42:54,872 trying to maybe engineer polymers 865 00:42:54,872 --> 00:43:00,744 to be more like nature, right? 866 00:43:00,744 --> 00:43:03,247 We were talking about Monday, if you 867 00:43:03,247 --> 00:43:05,049 want to try to solve the problems, 868 00:43:05,049 --> 00:43:06,483 try to do something big. 869 00:43:06,483 --> 00:43:08,552 And I gave an example of an ocean cleanup project. 870 00:43:08,552 --> 00:43:10,254 But the ocean cleanup project wouldn't it 871 00:43:10,254 --> 00:43:12,957 matter if we keep dumping more and more plastic in the oceans 872 00:43:12,957 --> 00:43:14,525 that doesn't degrade. 873 00:43:14,525 --> 00:43:17,428 All right, so we have to think very carefully about what 874 00:43:17,428 --> 00:43:18,729 kinds of plastics we can make. 875 00:43:18,729 --> 00:43:22,333 And the problem with cross-linking-- this 876 00:43:22,333 --> 00:43:25,369 gets to the negative side, right? 877 00:43:25,369 --> 00:43:27,171 And this is a way of categorizing 878 00:43:27,171 --> 00:43:29,173 these things that I've been talking about 879 00:43:29,173 --> 00:43:33,477 is if I cross-link it a lot, we call that a thermoset. 880 00:43:33,477 --> 00:43:35,079 We call that a thermostat. 881 00:43:35,079 --> 00:43:35,879 Why? 882 00:43:35,879 --> 00:43:39,249 Because when you heat it up, "thermo," it's set. 883 00:43:39,249 --> 00:43:43,821 So you mold it and then you've got your little cross-linker. 884 00:43:43,821 --> 00:43:45,889 You know, you pour your sulfur in 885 00:43:45,889 --> 00:43:48,926 and then it's got a very high number of cross-links. 886 00:43:48,926 --> 00:43:51,095 And you can see it there, right? 887 00:43:51,095 --> 00:43:54,031 And it gives you very strong material. 888 00:43:54,031 --> 00:43:56,900 But you can't break it back apart. 889 00:43:56,900 --> 00:43:59,336 So it's very hard to recycle. 890 00:43:59,336 --> 00:44:01,005 It's almost impossible. 891 00:44:01,005 --> 00:44:04,975 If you try to heat these back up, they just burn usually. 892 00:44:04,975 --> 00:44:06,677 And if you melt them, they're not usually 893 00:44:06,677 --> 00:44:09,013 that useful because they've got all this other chemistry 894 00:44:09,013 --> 00:44:11,081 and it completely messes with the polymer. 895 00:44:11,081 --> 00:44:12,950 On the other hand, a thermoplastic 896 00:44:12,950 --> 00:44:15,452 has this kind of plastic deformation. 897 00:44:15,452 --> 00:44:18,188 Think maybe like a garbage bag. 898 00:44:18,188 --> 00:44:20,724 So it can really stretch and stretch and stretch. 899 00:44:20,724 --> 00:44:24,028 But it doesn't have the cross-linkers. 900 00:44:24,028 --> 00:44:25,362 That's why you can do that. 901 00:44:28,032 --> 00:44:31,301 And so this is actually much easier, heat it up, melt it, 902 00:44:31,301 --> 00:44:32,569 and you can reuse it. 903 00:44:32,569 --> 00:44:36,040 So that's a recyclable polymer. 904 00:44:36,040 --> 00:44:39,643 But the issue there is it's very difficult 905 00:44:39,643 --> 00:44:43,447 to get all of the necessary mechanical properties 906 00:44:43,447 --> 00:44:45,649 from these thermoplastics. 907 00:44:45,649 --> 00:44:48,185 And so elastomers, which are maybe 908 00:44:48,185 --> 00:44:53,323 lightly cross-linked polymers that 909 00:44:53,323 --> 00:44:56,560 are above the glass transition temperature often, 910 00:44:56,560 --> 00:44:58,629 those are somewhere in between. 911 00:44:58,629 --> 00:45:02,199 But most of the time, even if you cross-link lightly, 912 00:45:02,199 --> 00:45:05,135 it's very difficult to recycle. 913 00:45:05,135 --> 00:45:08,238 And that's a really good problem because maybe we 914 00:45:08,238 --> 00:45:11,842 can come up with cross-link chemistries that 915 00:45:11,842 --> 00:45:13,844 are reversible, right? 916 00:45:13,844 --> 00:45:17,147 They can give us an opportunity to have 917 00:45:17,147 --> 00:45:19,416 the best of both worlds. 918 00:45:19,416 --> 00:45:20,851 It's elastic when we want. 919 00:45:20,851 --> 00:45:25,289 It's deforms when we want and it's solid when we want. 920 00:45:25,289 --> 00:45:28,058 Oh and by the way, you can degrade all those cross-links 921 00:45:28,058 --> 00:45:29,493 and reuse it. 922 00:45:29,493 --> 00:45:34,865 That's like one of the dreams in making environmentally friendly 923 00:45:34,865 --> 00:45:36,100 plastic. 924 00:45:36,100 --> 00:45:38,282 See you guys on Friday.