Morphology and crystallization kinetics of Rubber-modified Nylon 6 Prepared by Anionic In-situ Polymerization

2.1 Materials

Caprolactam (industrial grade) was provided by Baoding Mancheng Changsheng Plastic Factory. NaOH (reagent grade) was purchased from Jilin Haodi Chemical Reagent Sales Co., LTD. HDI urea diketone (Desmodur N 3400) with 21.8 wt% content of -NCO was provided by Bayer. Hydroxylterminated polybutadiene rubber (industrial grade, HTPB-I) with hydroxyl value of 1.31mmol/g was purcheased from Zibo Qilong Chemical Co., LTD. and its number-average molecular weight (M_n) is 1600-2300. Isopropanol (reagent grade) was provided by Jilin Haodi Chemical Reagent Sales Co., LTD. Toluene (analytically pure) was purchased from Jilin Haodi Chemical Reagent Sales Co., LTD. It was dryed, distilled and purified by 4A molecular sieve before use. Acetone (analytically pure) was purchased from Jilin Haodi Chemical Reagent Sales Co., LTD. Add KMnO₄ drop-wise to acetone until the liquid turns pink, then place it for 3-4d, following add anhydrous calcium sulfate for dehydration, distilling and purifying. Isopropanol (analytically pure) was provided by Jilin Haodi Chemical Reagent Sales Co., LTD. Bromocresol green indicator: Dissolve 0.1g bromocresol green in 100mL ethanol with the volume fraction of 20%; 0.1mol/L acetone/di-n-butylamine solution: Put 2.60g di-n-butylamine in a 200mL volumetric flask, and add acetone to the scale to dissolve di-n-butylamine, then shake, and store in dark place for future use. Prepare 0.1mol/L HCl standard solution by conventional method.

2.2 Specimen Preparation

Preparation of polybutadiene rubber activator terminated by acyl caprolactam: Put 310g HDI urea diketone and toluene solvent in a 3-mouth flask, slowly add 80g HTPB-I rubber prepolymer, heat to 80^∘C, and react for 3h until hydroxyl is reacted completely. 113g caprolactam was melt, dried under vacuum to remove moisture, and then add to the above system. The mixed system was reacted at 100^∘C for 2h, and then heat to 130^∘C to react until the reaction was completed. The judgment criterion for the end of the reaction is that isocyanate and di-n-butylamine-acetone completely react. Afterwards, add isopropanol solvent to precipitate macromolecule of the rubber grafted acyl caprolactam, place overnight, and wash for several times with isopropanol to remove the small molecular substances. Remove toluene and isopropanol from the obtained product through the reaction flask and finally get acyl caprolactam-terminated HTPB-I activator.

Preparation of rubber copolymerization modification nylon 6: Accurately weigh caprolactam (accounting for 0.8 of the total monomer mass) and rubber activator (accounting for 0.1-0.2 of the total monomer mass), mix uniformly, and remove the tracewater by the vacuumat 130^∘C. Then add NaOH (accounting for 0.008 of the quality of total monomeric substance) catalyst in the remaining caprolactam monoer (accounting for 0.2 of the total monomer mass) and react at 130^∘C to generate the active anion. Afterwards, mix uniformly, cast at an ordinary pressure, maintain the polymerization reaction for 30 min in the thermal insulation condition of 180^∘C, and gradually cool and demould to get the product.

2.3 Characterization

3.1 Representation of prepolymer of the rubber activator and analysis of rubber-modified nylon

Hydroxyl-terminated polybutadiene rubber (HTPB) having high activity, can react with isocyanate without catalyst, and is terminated by caprolactam. Figure 1 shows the infrared analysis of all samples. In Figure 1a of curve spectrum, the absorption peaks at 3080cm⁻¹ [29], 3020cm⁻¹, 1830cm⁻¹, 1641cm⁻¹, 1424cm⁻¹, 1000cm⁻¹, 912cm⁻¹, 690cm⁻¹, and 912cm⁻¹ are relatively strong, which suggests that HTPB used in the experiment contains many 1, 2 addition structures [30]. The absorption peaks at 3020cm⁻¹ and 970 cm⁻¹ (wagging vibration of proton on anti-form olefinic carbon) are relatively strong, which implies that HTPB also contains many anti-form 1, 4 addition structures. The absorption peaks of cis-form 1, 4 addition structure are located at 3020cm⁻¹, 1660cm⁻¹, 1410cm⁻¹ and 730cm⁻¹, but the absorption peaks areweak, which suggests that HTPB contains few cis-form 1, 4 addition structures; and 3356cm⁻¹ indicates the characteristic spectra of -OH and HTPB [31]. In Figure 1b, the stretching vibration of -NH group is located at 3365cm⁻¹ and 1701cm⁻¹ [29], and the bending vibration of N-H is located at 1539cm⁻¹ [32]. Generally, the stretching vibration absorption of carbonyl of CONH is located at 1768cm⁻¹ [30], which is the characteristic spectra of the stretching vibration of NHCO. The characteristic absorption peak of C=O on tertiary amide is located at 1658cm⁻¹ [33], which shows that caprolactam has reacted with -NCO in prepolymer. No absorption peak of -NCO is located at 2270cm⁻¹, suggesting that -NCO has been closed [30]. It shows that HTPB reacts with HDI urea diketone, and caprolactam is used for terminating successfully. Figure 2 shows the infrared spectrum of nylon and rubber-modified nylon. As can be seen from Figure 2, the position of characteristic absorption band of amide group from nylon 6, such as N-H stretching vibration at 3300cm⁻¹, amide absorption band I at 1631cm⁻¹, amide absorption band II at 1536cm⁻¹, amide absorption band III at 1259cm⁻¹ [29, 34] and methylene stretching vibration peak at 2931cm⁻¹ and 2850cm⁻¹ [35], is very obvious. Moreover, one more reflection peak of polybutadiene rubber is provided at 966cm⁻¹, and as the dosage of HTPB is increased, the peak is gradually strengthened, which indicates that active prepolymer generates the copolymer of Nylon 6-HTPB-Nylon 6 by virtue of anionic polymerization.

Figure 1

HTPB prepolymer terminated by N-acyl caprolactam and infrared spectroscopy of HTPB prepolymer Note: a: HTPB; b: HTPB terminated by N-acyl caprolactam.

Figure 2

Infrared spectrum of nylon and rubber-modified nylon.

3.2 Morphology and impact section of rubber-modified nylon

During the polymerization reaction, as the molecular chain of nylon is generated, the microphase separation occurs because the solubility parameter of nylon 6 is diffenent from that of rubber phase. The rubber phase is uniformly distributed in the nylon matrix in situ,which is similar to the two-phase distribution of Styreneic Block Copolymers (SBS). Figure 3 shows the SEM micrographs of cryofracture surfaces after etching. As shown in Figure 3(a), no any change is provided on the surface through the toluene solvent etching. Figure 3(b) shows the section morphology of modified nylon with rubber content of 5%, from which it is found that it is hard to see the etching mark of the rubber phase on the section. This may be because the rubber is very uniformly distributed in the matrix through the covalent bonding, and the content of the rubber phase is too low, so it is insufficient to form the microphase. Figure 3(c) shows the section morphology of the modified nylon with the rubber content of 10%. The etching mark of the rubber phase on the section is very slightly, When the content of the rubber phase is increased, it is too late for part of rubbers to react, or part of rubber cannot be grafted on the active group during rapidly polymerization, resulting to form the microphase. Figure 3(d) shows the section morphology of the modified nylon with the rubber content of 15%. The etching mark of the rubber phase on the section is obviously, and the increase in the rubber phase content makes the size of microphase increase further. Moreover, many etching pits are presented after the toluene etching, but the size is very small and uniform, which indicates that the rubber can form the microphase with uniformly distributed in the matrix. Figure 3(e) presents the section morphology of the modified nylon with the rubber content of 20%. The etching mark of the rubber phase on the section is increasingly obvious, and the size is increased and nonuniform, which indicates that the content of the rubber failing to participate in the polymerization is further increased, and the big microphase is formed, so they cannot be distributed in the matrix uniformly.

Figure 3

SEM micrographs of cryofracture surfaces after etching: a: Nylon 6; b: Nylon 6/HTPB=100/5; c: Nylon 6/HTPB=100/10; d: Nylon 6/HTPB=100/15; e: Nylon 6/HTPB=100/20.

The roughness of the impact section can reflect the situation of crack propagation of nylon 6 and modified nylon 6. Figure 4 shows the SEM micrographs of impact section of nylon 6 and rubber-modified nylon 6. It can be seen from Figure 4 that the molecular chain of the pure nylon 6 is relatively structured and has relatively high crystallinity. The energy cannot be consumed timely under the impact of external forces, and the craze is produced and rapidly expanded into the big crack, then, the resin is fractured soon and shows brittle. Therefore, the impact section is relatively smooth. When the rubber content in the resin is increased, the impact section is much rough comparing with the that of pure nylon 6. Especially, when the rubber content is higher than 15%, the resin shows obvious ductile fracture. Therefore, it can be concluded that the molecular chain can be loose and is subjected to the shear yield under the effect o the external force, so that the impact energy is dissipated.

Figure 4

SEM micrographs of impact section of nylon 6 and rubber-modified nylon 6.

3.3 Crystalline structure of the rubber-modified nylon

Nylon 6 is the polycrystalline crystalline polymer, and takes on α and γ crystal structures according to the different crystallization conditions. α is the relatively common crystal form, in which the molecular chain in the crystalline region is completely stretched. Amide chain segment and methylene chain segment are located in the same plane, and are connected to each other through hydrogen bond. As a result, the plane slice layer is formed, which is generally generated in the high crystallization temperature. γ crystal form is metastable, in which the hydrogen bond direction is close to the plane of the vertical framework, which shows the kilted slice layer. γ crystal form generally generated in the low temperature. X-ray diffraction of α crystal form has two peaks near 20.5^∘ and 23.6^∘, which belongs to the (200) and (002) crystal faces of α crystal form, respectively. γ crystal form only has the single peak at 2θ = 21.5^∘, which belongs to the (200) and (101) crystal faces of γ crystal form. Figure 5a shows the WAXD spectrogram of nylon 6 pellets polymerized and modified by liquid rubber with different contents in situ. It can be seen that two peaks of α crystal form are presented near 20.5^∘ and 23.6^∘, and no new diffraction peak is found, which suggests that the introduction of the rubber prepolymer does not change the crystalline structure of the nylon 6 after the polymerization at high temperature. Furthermore, the position of the diffraction angle of α crystal form of nylon 6 is not basically changed when the liquid rubber content is in the range of 0-15%, and when the rubber content is up to 20%, the spectral peak of crystal face (002+202 and 200) moves towards the high angle. The reason may be that the interplanar spacing of the molecular chain of nylon 6 is reduced due to the orientation shrinkage of rubber occurs, and the strength of the diffraction peak of α crystal form is decreased caused by the mixing of the rubber. Therefore, the addition of the rubber affect the formation of α crystal form. In general, the rubber has big influence on the crystal form of nylon 6, thereby, keeping the original nature of nylon 6 is important. Figure 5b shows the WAXD spectrogram of nylon 6 pellets polymerized and modified by liquid rubber with different contents in situ after melting in high temperature and quick-cooling. It can be seen that many γ crystal forms are provided on the molecular chain of nylon 6 when quick-cooling, which indicates that it is too late for the molecular chain to arrange in order, thus, forming the metastable crystal form. As the rubber content is increased, the diffraction peak of α crystal form is obviously enhanced, indicating that the crystal form of nylon 6 changes from strong γ crystal to crystals with γ and α crystal form coexisting, and then changed to strong α crystal form. These results suggest that the activity ability of the molecular chain becomes strong due to the existence of the flexible chain segment of the rubber, and the molecular chain can be arranged in order in shorter time, resulting in the formation of the relatively stable α crystal.

Figure 5

WAXD spectrogram of nylon 6 and rubber-modified nylon 6.

3.4 Nonisothermal crystallization kinetics

The crystallization properties of nylon 6 and rubber-modified nylon 6 resin are different as the processing temperature is different. This paper studied the nonisothermal crystallization kinetics by virtue of Jeriorny method [36]. Figure 6 gives the DSC curve of nylon 6 and modified nylon 6 in the nonisothermal crystallization process. It can be seen that the temperature for starting crystallization gradually shifts towards the lower temperature, and the crystallization peak broadens obviously as the cooling rate is increased. Because the cooling rate is increased, the chain segment is crystallized at the lower temperature, and the activity ability of the chain segment decreases. Therefore, only part of chain segments can enter the crystalline phase and become the most stable crystal. As the cooling rate is increased, the degree of crystal imperfection increases, so the crystallization temperature range increases and the crystallization peak become wider. The temperature for starting crystallization of the rubber-modified nylon 6 moves to the low temperature, suggesting that the nylon 6 chain segment enters the unit cell, reducing the nucleation due to the flexible rubber chain segment.

Figure 6

DSC curve of nonisothermal crystallization of nylon 6 and rubber-modified nylon 6.

Figure 7 shows the DSC heating curves of nylon 6 and rubber-modified nylon 6. It can be seen from Figure 7(a) that the melting point temperature (T_m) of nylon 6 subjected to the anionic polymerization is 211.00^∘C, and the T_m of the modified nylon presents the decreases as the liquid rubber activator is added. For example, the T_m of modified nylon 6 with the rubber content of 20% is 202.67^∘C. It can be seen that the rubber molecules increase the flexibility of nylon chain, and also reduce the hydrogen bond density of nylon 6. At the same time, the proportion of the amorphous region is increased, leading to the limitation and interference of the growth of spherulites. Therefore, the growth of spherulite of the polymer is reduced. It can be seen from Figure 7(b) that nylon 6 and modified nylon 6 can crystallize at cooling rate of 10^∘C/min. Because in the follwing second heating curves, the melting peak of the crystal appeare. It can be seen that the T_ms of nylon 6 and modified nylon 6 slightly increase, which indicates that the chain segment has sufficient time to arrange into the stable state when cooling.

Figure 7

DSC heating curves of nylon 6 and rubber-modified nylon 6.

The calculation formula (1) of the relative crystallinity X(T) in the nonisothermal crystallization process is as follows [37]:

where X(T) is the relative crystallinity when the temperature is T, and dH_c/dT is the heat flow rate of crystallization at T.

Figure 8 shows the curves of relative crystallinity against time for all the resin. It can be seen that the curves are obvious S type. When the relative crystallinity is within 10%, the nonisothermal crystallization of polymer starts, and the nucleation rate is very small. At this time, the crystallization rate is controlled by the nucleation process, and the change in the relative crystallinity with temperature or time is slower. As the nucleation rate becomes faster, the crystal growth becomes quickly, the crystallization rate increases rapidly, and the relative crystallinity changes faster with time or temperature. When the relative crystallinity is above 90%, the nucleation rate of the crystal is still large, but the crystal growth rate gradually becomes slow, resulting in a slower change of the relative crystallinity with temperature and time.

Figure 8

Plots of the relative crystallinity with crystallization time.

In the nonisothermal process, the time-temperature transformation will be carried out by using the formula:

where t is the crystallization time, T₀ is the initial crystallization temperature, T is the crystallization temperature, and R is cooling rate. The relation curve of X(t) and t can be obtained from the time-temperature transformation, and the half-crystallization time t_1/2 can be directly calculated from the curve. The DSC crystallization curve of varying temperature at constant rate can be taken as the isothermal crystallization process. Therefore, the relevant parameters can be corrected, and Avrami equation is popularized and applied to the nonisothermal crystallization process. In other words, Avrami equation can be expressed as Formula (3) [38, 39].

where n is Avrami index, which is related to the nucleation mechanism and growth mode of the crystal, and Z_t is the rate parameter in the nonisothermal crystallization process. Jeziorny proposes that the rate parameter Z_t should be corrected, so that Avrami equation can be applicable to describe the nonisothermal crystallization process, namely, Formula (4) [36]:

where Z_c is the rate constant after correction; and φ is the heating or cooling rate.

The crystallization kinetics curves of samples are obtained by plotting lg [− ln (1 − X_t)] against lgt and are shown in Figure 9. It can be seen from Figure 9 that lg[− ln(1 − X_t)] has good linear relation with lgt under different cooling rates when the relative crystallinity is 10%-90%. n and Z_c are calculated from the slope and intercept of the fitting curves and are listed in Table 2. It can be seen from Figure 9 and Table 2 that n value of nylon 6 is 3.97-4.22, suggesting that nylon 6 crystal grows mainly in a three-dimensionally during the nonisothermal crystallization process. n value of rubber-modified nylon 6 is 2.19-3.65, indicating that its growth mode is the coexistence of the two-dimensional discoid growth and three-dimensional spherulitic growth. In addition, a certain deviation is provided when entering the late period of the crystallization. This is because the crystalline grains are contacted to each other, and the crystal stops growing in the contact direction, while the crystal continues to grow in the direction without collision, resulting in the imprvement of the incomplete part. in the early stage of crystallization, the crystallization rate is controlled by nucleation, and the crystal growth rate changes with the time. In the late stage of crystallization, the crystallization process is controlled by diffusion, and the growth rate becomes slower.

Figure 9

Avrami plots of nylon 6 and rubber-modified nylon 6.

The nonisothermal crystallization parameters of the nylon and modified nylon at different cooling rates are listed in Table 2. It can be seen from Table 2 that the temperature at which crystallization starts (T_S) and crystallization peak temperature (T_max) of the rubber-modified nylon 6 and nylon 6 both move to a low temperature direction as the cooling rate continuously increases, suggesting that the nonisothermal crystallization process of the nylon is controlled by nucleating. At a low cooling rate, the chain segment has sufficient time to arrange into the stable state and further nucleate to form the microphase. When the cooling rate is continuously increased, the time for chain segment stacking becomes shorter, so, it is difficult to enter the unit cell with the stable state, resulting in delayed nucleation and a decrease in initial crystallization temperature. The half-crystallization time (t_1/2) becomes shorter as the cooling rate is increased, which indicates that the nucleation rate increases due to shortened crystallization time caused by cooling. On the other hand, the t_1/2 decreases as the rubber content is increased, suggesting that the rubber promotes the crystallization of the nylon 6, and accelerates the crystallization rate. As the cooling rate is increased, the crystallization rate constant Z_c of nylon 6 increases, and t_1/2 obviously shortened. This is because that the increase in cooling rate increases the degree of supercooling and accelerates the transition of the system from the melt state to the crystalline state, thus increasing the crystallization rate. As the rubber content is increased, the t_1/2 of rubber-modified nylon 6 is shortened at the same cooling rate, indicating that the addition of the rubber promotes the crystallization of nylon 6 matrix.