Influence of Material Ratio on Controlled Cross-linking Process

Influence of Material Ratio on Controlled Cross-linking

Process

Fangxing Li, Ruimin Sun, Jun Chen, Dongping Liu

Department of Chemistry, Nankai University, Tianjin, 300071

E-mail: lifangxing@https://www.360docs.net/doc/5613729943.html,

Abstract

There exist intra- and intermolecular reactions in a controlled cross-linking system, and whether cross-linking process and molecular weight of product can be controlled depends on the two reactions. Soluble cross-linked macromolecule is synthesized by changing material ratio resulting in that the number of intermolecular reaction is controlled to a certain extent and molecular weight can’t increase illimitably. Though material ratio is a common chemical reaction condition, it has been proved that it can influence and control chain growth in cross-linking process in the paper. Keywords: inter- and intramolecular reactions, cross-linking process, material ratio.

Introduction：Linear, branched, star, combed, and cycled macromolecules all exhibit plain zigzag structures when fully stretched out, though their configurations are quite diverse. However, soluble cross-linked macromolecule is 3-d network structure, based on which there are more space to design molecule structure and novel material. Moreover, soluble cross-linked macromolecule has special characteristic. Therefore, it is an interesting study to investigate the control of cross-linking process. People have been able to synthesize soluble cross-linking macromolecule only using special experiment technique for a long time, but the question controlling cross-linking process hasn’t been solved.[1~9]

We have present the mechanism controlling cross-linking process in our paper in China Sciencepaper Online. On the base of the mechanism, a polymerization system, in which chain growth can be controlled, is designed, and soluble cross-linked macromolecule is formed [10]. There exists a pair of competitive reactions, viz, intra- and intermolecular reactions in the controlled cross-linked polymerization system [11~15]. Intramolecular reaction can’t increase molecular weight, and the more the number of intramolecular reaction is, the lower molecular

weight is. On the contrary, intermolecular reaction can increase molecular weight, and the more the number of intermolecular reaction is, the higher molecular weight is. Consequently, to control cross-linking process is to control the proportion of intra- and intermolecular reactions. It is worth to think whether the proportion of intra- and intermolecular reactions can be controlled using common chemical reaction conditions, such as: material ratio, concentration, catalyst, temperature, and so on.

Intra- and intermolecular reactions exist in the same polymerization, so it is important whether their reaction rates can change differently after the above conditions change. In fact, there is complete different environment in intra- and intermolecular reactions, their answers to reaction conditions changing are different. In the paper, it has been found that cross-linking process can be controlled by changing material ratio to limit the number of intermolecular reaction.

1. Experiment

1.1 Synthesis of Polyvinyl Butyral (PVB)

1.1 Synthesis of Polyvinyl Butyral (PVB)

The polymerization was carried out in a three-neck flask of 5000 mL equipped with a stirring bar. 260g of PV A was added firstly into 3000mL water and was dissolved in 80o C. After mixture was cooled to room temperature, 100mL of n-Propanoic acid and 10mL of 36% hydrochloric acid were added, and 168mL and 252mL of n-butyl aldehyde respectively, thus, two kinds of samples would be produced. Reaction mixture solution became cloudy after 30 mins, and it increased to 65o C 2h later and continued to react for another 4h, the solution became clear again. The solution was washed to be neutral after experiments finished. At last, hydroxyl number of samples was examined after samples were dried. Parameters of the two samples are listed in Table 1.

Table 1 Parameters of PVB samples

No. A B Hydroxyl number 132.0 58.4

Functionality a197 86 a: average number of –OH on a molecular chain.

1.2 Synthesis of Soluble Cross-linked Macromolecules

1.0265g of PVB with 132 hydroxyl number was first dissolved in 10mL DMF in a reaction vessel, then 0.1523g MDI dissolved in 5mL DMF in a Erlenmeyer was added into the vessel, and the

Erlenmeyer was washed with 5mL DMF for three times and it was also poured into the vessel. Gelation occurred after a certain time, and gelation time was recorded exactly. In the paper gelation solution was called gel. Gelation solution continues to react for 4h at room temperature and at 80o C, respectively. A serial of gels were formed according to different molar ratio of MDI and OH.

1.3. Solubility estimation of gel

1g of the gel synthesized above was added into 30mL DMF, and the mixture was placed and shaken unceasingly at room temperature, if the gel couldn’t be dissolved after 4 hours, the mixture was refluxed at 156 o C for 12h. If the gel still couldn’t be dissolved, the gel was regarded as insoluble; otherwise, it was looked as soluble.

2．Results and Discussion

2.1 Polymerization

Reactions in this paper are shown in Scheme 1:

CH2CH

O CH2CH

O

CH2CH

OH

CHCH2CH2CH3CH2CH

OH

+

A

CH2

OCN NCO

CH2CH

O

CH2CH

O

CH2CH

CHCH2CH2CH3

CH2CH

OH

OCNH R

O

NCO

B

R=CH2

Scheme 1 Prepolymer Synthesized from PVB and MDI with -OOCNH-, -OH and -NCO Groups

on the Chain

A is the molecular formula of PV

B that contains –OH and –OCH(CH2CH2CH3)O- groups on its chain; B is the prepolymer from PVB and MDI. There are –OOCNH- ,–NCO –OH and –OCH(CH2CH2CH3)O-groups on the chain of prepolymer, and –NCO can react with hydrogen atom in –OOCNH- and –OH, so B is the prepolymer containing a pair of competitive groups. In Figure 2, there are intra- and intermolecular reactions in the controlled cross-linking polymerization.

1-1 1-2

Fig 1 Sketch for Reaction of PVB and MDI

? : MDI molecule

: -NCO group

? : -OCONH- group

2.2 Controlled cross-linking process and ICM’s structure.

197PVB-0.25-a 197PVB-0.25-b 197PVB-0.25-c

197PVB-0.25-d 197PVB-0.2-a 197PVB-0.2-b

Figure２TEM Pictures of 197PVB Samples

In Figure２, there are TEM pictures of ICM formed in 197PVB-0.25 and 197PVB-0.2 experiments. As is shown in TEM picture, each ICM is composed of lots of globules, showing that the globules are formed at the early stage in polymerization process, and ICM is formed by remanent functional groups on the surface of globules continuing to react. It means that there exist two kinds of reaction processes in the controlled cross-linking polymerization. In the first process, prepolymers react with each other to form greater and greater coils [11], and cross-link degree increases gradually with intra- and intermolecular reactions in coils; molecular weight increases

by intermolecular reaction, at last, globules with certain molecular weight and steady structure are formed. In the second process, remanent groups on the surface of globules continue to react with each other, and some globules are linked together to form ICM. It can be seen that the two processes can be looked as special chain growth in the controlled cross-linked polymerization process and the link of ICM is composed of globules. Therefore, ICM’s segment is different with that of linear polymer. Thus, ICM in the paper must be different with linear polymer.

There exist two polymerization processes in the controlled system, so ICM has two kinds of network structures. C-C link is formed in the first process, and in the paper, the network formed by C-C links is called chemical network; network structure formed by globules is called physical network.

Table ２Experimental Results of 197PVB

No. PVB (g) MDI

(g) M c NCO:OH (mol:mol)

DC (%) Gelation time (h)a Solubility 197PVB-0.50 1.0265 0.1523 644 1:2

12.920.35 －b 197PVB-0.33 1.1237 0.1107 928 1:3

9.75 0.65 ++c 197PVB-0.25 1.1613 0.0837 1245 1:4

6.72 0.90 ++ 197PVB-0.20 1.1394 0.0708 1441 1:5

5.81 1.8 +d 197PVB-0.17 1.0140 0.0497 1773

1:6 4.67 NO a : Gelation time. b : －Gel is insoluble. c : ++ Gel is soluble when refluxed in DMF. d : + Gel is soluble at room temperature.

197 and 0.5 in 197PVB-0.5 are functionality of PVB and molar ratio of NCO and OH respectively.

2.3 Linking of Pre-ICM

In the above section, it has been discussed that ICM is formed by globules. In the paper, therefore, the globule is called pre-ICM. It can be found from Figure 1 that there are two kinds of linking methods about ICM. One is linearly-linking, for example, 197PVB-0.2-a in Figure 2. There is also linearly-linking in 197PVB-0.25-b, but it is less obvious than that in 197PVB-0.2-a. The other is accumulating-linking, such as: 197PVB-0.25-b and 197PVB-0.25-c. Maybe the different linking methods are relational with amount of the remanent groups on the surface of globules. From Figure 2, it can be known that pre-ICM has large molecular weight, thus, it is difficult to link two globules by one single bond. If there is only one single bond to link two globules, the single bond would rupture because it couldn’t support the strain between two

globules. Thereby, there must be many single bonds to link globules so that globules can be linked steadily. Like these bonds is called “bond-cluster” in the paper. If there are not enough remanent groups on the surface of globules, globules will be linked in linearly-linking by two “bond cluster”; if there are enough remanent groups, globules will be linked in accumulating linking. The figures of 197PVB-0.25-a and 197PVB-0.25-b show almost nubby structure, maybe because there are too many remanent groups on the surface of the globules to lead to accumulate them closely.

2.4 Effect of molar ratio of NCO and OH on cross-linking process

Effect of molar ratio of NCO and OH on cross-linking process is the effect of material ratio on cross-linking process. In Table 2, experimental results of 197PVB are listed. From Table 2, it can be known that the gel in 197PVB-0.5 experiment, in which molar ratio of NCO: OH is 0.5, is insoluble, showing the number of intermolecular reaction is too more, resulting in all materials react together, or only one macromolecule is formed. Molar ratio of NCO: OH in 197PVB-0.33 experiment decreases to 0.33, its product is soluble when refluxed in DMF, showing the number of intermolecular reaction is not too more. It can be concluded that if amount of MDI used decreases, the number of MDI in outside coils also decreases, and the number of intermolecular reaction decreases, thus, soluble macromolecule with cross-linked structure is formed. But the gel in 197PVB-0.33 experiment is soluble only when refluxed at 156oC in DMF. Molar ratio of NCO: OH in 197PVB-0.25 experiment is 0.25, but the gel formed is soluble only when also refluxed in DMF. It means that amount of MDI used decreases, but the number of intermolecular reaction is still enough to form macromolecule with large molecular weight. Molar ratio of NCO: OH in 197PVB-0.2 experiment is 0.2, and the gel formed in the system is soluble in DMF at room temperature, which shows though the gel have cross-linked structure, its entanglement isn’t stronger than that of gels in 197PVB-0.33 and 197PVB-0.25. Molar ratio of NCO: OH in 197PVB-0.17 is 0.17, gelation don’t occur during experiment period, which shows that amount of MDI used is too little so that the number of intermolecular reaction is too little and molecular weight is not enough large. Thus, cross-linking process is controlled. Therefore, in the controlled cross-linked polymerization, soluble cross-linked macromolecule can be synthesized by changing material ratio to limit chain growth illimitably.

2.5. Effect of functionality on cross-linking process

Functionality is the number of groups on molecular chain. Take 197PVB and 86PVB for

example, there contains 197 –OH (the other groups aren’t contained) on average every molecular chain of 197PVB, while there contains only 86 –OH on the chain of 86PVB. Functionality difference of 197PVB and 86PVB leads to the different experimental results. Experimental results of 86PVB are listed in Table 3. Table 3 Results of Samples in 86PVB Reacted with MDI

No. PVB (g) MDI

(g) M c NCO:OH (mol:mol)

DC (%) Gelation time (h) Solubility 86PVB-1.20 1.0243 0.1610 697 1.2:1

13.58 1.2 － 86PVB-1.00 1.0657 0.1404 718 1:1 11.4 2.7

++ 86PVB-0.75 1.0058 0.0993 929 0.75:1 8.99 3.8

+ 86PVB-0.50 1.0656 0.0700 1352 1:2 6.16

NO In Table 3, molar ratio of NCO: OH in 86PVB-1.2 experiment is 1.2, the gel formed is soluble. The molar ratio is the lowest in the serial experiments in which the gel formed are insoluble. While, in the serial experiments of 197PVB, the lowest molar ratio is 0.5 when the gel formed is insoluble. The lowest molar ratio is an important parameter comparing the two serials of experiments in the paper. Molar ratio of NCO: OH in 86PVB-1 experiment is 1, the product is soluble when refluxed in DMF, and the molar ratio is the highest in the serial experiments in which the gel formed are soluble. The highest molar ratio is also an important parameter comparing the two serials of experiments in the paper. In the two serial experiments, due to their different functionality, the two parameters are different respectively. In the serial of 197PVB experiments, because –OH is more on molecular chain, -OH is more on the surface of coils, intermolecular reaction can occur when only little MDI exists outside coils. In the serial of 86PVB experiments, the number of –OH obviously decreases, only when more MDI is added, intermolecular reaction can occur, resulting in that the gel can be insoluble. The difference shows that the less functionality on the chain is, the easier cross-linking process is controlled. Take 86PVB-1.2 experiment for example, the number of NCO is more than that of OH, superfluous NCO groups will react with hydrogen atom in –OOCNH-. Comparing the experimental results in Tables 2 and 3, it can be found that functionality have various influence to cross-linking process. Conclusions: In the controlled cross-linking polymerization system, cross-linking process can be controlled and soluble cross-linked macromolecule can be synthesized by changing material ratio.

Moreover, there exist critical molar ratio of NCO and OH, if molar ratio of NCO: OH is more than the critical molar ratio, the gel formed will be insoluble; only when molar ratio of NCO: OH in reaction system is less than the critical molar ratio, the gel formed can be soluble.

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