Enhancing Polymer Composite Analysis with Advanced Techniques

Abstract

The compatibility of polymer composites is critical for determining their performance and application. This essay explores various methods to ascertain the compatibility of polymer blends, focusing on thermodynamics, morphology, and phase behavior. The Gibbs free energy of mixing (ΔG) serves as a thermodynamic indicator of miscibility, while Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) among other techniques offer insights into the morphological behavior of polymer blends.

Introduction

After polymer are blended, several methods to find the compatibility of polymer composites will be conducted.

In thermodynamics view, when the mixture is miscible, the Gibbs free energy of mixing (ΔG) is less than 0. In terms of morphology, it can be divided by two categories which are Interface and phase behaviour. In order to get morphology behaviour of mixture, the number of glass transition temperature are used. Normally, most common method to determine the glass transition temperature is DSC. When DSC is used for measuring glass transition temperature, if the mixture is miscible, there is one glass transition temperature of steep curve on DSC trace.

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Moreover, if there is two glass transition temperature on DSC trace, it can say that this mixture is immiscible. To obtain glass transition of polymer blends, other methods also can be used such as DMA (Dynamical Mechanical Spectroscopy), Dielectric Relaxation spectroscopy and Dilatometry.　When it comes to partially miscible polymer blends, MDSC are able to use. Normally, when the two polymers mix together there are three phases A-rich phase, B-rich phase and AB interface. It shows in Figure 2.

Due to every polymer has different heat capacity (ΔCp), we can observe the difference of their heat capacity by MDSC (given in Figure3). It is possible to determine the weight fraction of Since there are equations for miscible blends.

Tg = wATg1 – wBTg2

ΔCp = wAΔCpa＋ｗBΔCpB

ｗA＋ｗB＝1

Where,

Tg : Glass transition temperature

w : Weight fraction

ΔCp : Heat capacity

Subscript A: polymer A

Subscript B: polymer B

In order to calculate the weight fraction of interface for partially miscible polymer (Whole system), ΦA and ΦB are calculated by equation.

Weight fraction (ΦA, ΦB) = ΔCp (measured by MDSC) / ΔCp (theoretical calculation)

ΔCp (theoretical calculation) can be determined by equation 1,2 and 3. Using equation, Weight fraction of interface can be calculated.

Weight fraction of interface = 1- wa -wb

In order to describe the methods of calculation for weight fraction clearly, the example of question and answer shows below. Table1 will be given when the calculation is needed.

Using equation (1) to calculate the weight fraction in A rich phase,

WA ×19 + WB ×105 = 36

From equation (3), WB = (1-WA)

WA ×19 + (1-WA) ×105 = 36

19WA × + 105 – 105 WA = 36

86 WA = 69

In A rich phase, weight fraction of A and B are WA =80% WB = 20%

Using equation (2) to calculate the theoretical heat capacity in A rich phase,

ΔCp = 0.8 × 0.41 + 0.2 ×0.35

= 0.398 (J/g/oC)

The same things can do with B rich phase

In B rich phase

WA ×19 + (1-WA) ×105 = 82

86 WA = 23

In B rich phase, weight fraction of A and B are WA = 26.7% and WB = 73.3%

Using equation (2) to calculate the theoretical heat capacity in A rich phase

ΔCp = 0.267 × 0.41 + 0.733 ×0.35

= 0.366 (J/g/oC)

Using equation (4) to calculate how many A phase and B phase in whole system.

Weight fraction (ΦA) = 0.16 / 0.398

Weight fraction (ΦB) = 0.13 / 0.366

Weight fraction of interface = 1- ΦA - ΦB = 0.243

For these calculations and the result of MDSC, it is possible to calculate the weight fraction of each polymer for mixture.

According to another articles, it is possible to determine the characteristics of crystallization behaviour of mixture from DSC trace of melting peak and melting temperature.

They mixed LDPE and PA6 together with compatibilizer. As for compatibilizer, hydroxyl terminated polybutadiene (HPB) were synthesized with ε- caprolactam and used with LDPE/PA mixture. They synthesized different structure of HPB due to find the best compatibilized structure of HPB. Table2. shows the list of compatibilizer which they made.

They conducted SEM morphology test and tensile test from these experiments, C3 shows the highest compatibility with LDPE/PA and followed by C2, C1. After that they searched the characteristics of diblock copolymer by DSC. Figure 4 shows that DSC trace of (a) HPB (b) PA6 own, (c) C3 and (d) C2. Compared with (c) C3 (d) C2, C2 shows more broad peaks. C2 contains 33% of 1,2 polybutadiene in their structure and C3 contains 12% of 1,2 polybutadiene in their structure. 1,2 polybutadiene has less compatibility with low density polyethylene compared to a hydrogenated 1,4 polybutadiene. When 1,2 form the microstructure, they are influenced by polarity of medium which used for synthesis. They concluded that if the portion of 1,2 polybutadiene is higher, crystallization and melting peak become broader. In other words, crystallinity will be decreased with the content of 1,2 increased.

In terms of melting temperature, the other author searched about the relation ship between grafting degree of compatibilizer with melting temperature. They used grafted maleic anhydride as a compatibilizer to mix LDPE and PA6 together. And they increased grafting degree of LDPE-g-MAH from 0.2% to 5.1% in order to determine the crystallinity behaviour of mixture from DSC. Figure 5 shows the DSC trace of melting peak for different degree of grafting LDPE-g-MAH. If the degree of grafting is increasing, the melting temperature became lower. It is because of the MAH group can decrease the crystallinity of polymer. This phenomenon also proved that another article with PP and PA mixture with compatibilizer (Succinic anhydride grafted PP) [27]. From Figure5, two endotherm peaks are obtained with the grafted LDPE which are 83℃ and 104℃. And it shows lower crystallinity of pure LDPE.

Using these methods, the morphology of interface can be observed. However, there is drawback to use Tg for determining the compatibility of mixture which is sensitivity of equipment. Since small amount of material are used for DSC, typically ~10mg, if the concentration of polymer blends is under 10 % it is difficult to get the accurate data from DSC. [25]　For this reason, using multi methods for analysing compatibility is necessary.

Conclusion

The compatibility of polymer blends significantly influences their properties and applications. Advanced techniques like DSC, DMA, and MDSC provide comprehensive insights into polymer miscibility and morphology. Multi-method analysis is essential for accurate compatibility assessment due to the limitations of single techniques, especially in blends with low concentration polymers.