Non-isothermal crystallization kinetics of reactive microgel/nylon 6 blends☆

Min He ,Siqi Zong ,Yahuan Zhou ,Huaibing Guo ,Qingchun Fan ,*

1 School of Chemistry and Environmental Engineering,Wuhan Institute of Technology,Wuhan 430074,China

2 School of Chemical Engineering and Pharmacy,Wuhan Institute of Technology,Wuhan 430074,China

Keywords:Reactive microgel Nylon 6 Non-isothermal crystallization kinetics Crystallization activation energy

ABSTRACT The non-isothermal crystallization kinetics of reactive microgel/nylon 6 blends was investigated by differential scanning calorimetry(DSC).The Mo equation was employed to analyze the non-isothermal crystallization data.The crystallization activation energies were also evaluated by the Kissinger method.The results show that the crystallization onset temperature(T onset)and crystallization peak temperature(T p)decrease with the increase of the content of reactive microgel,while ΔT(T onset–T p),the crystallization half-time(t1/2)and the crystallization enthalpy(ΔH c)increase.The required cooling rates of blends are higher than that of neat nylon 6 in order to achieve the same relative crystallinity in a unit of time.The crystallization activation energies of the reactive microgel/nylon 6 blends are greater than those of the neat nylon 6.When the content of reactive microgel is 30%,the relative crystallinity(Xt)reaches the maximum.

1.Introduction

Nylon 6 is an important class of crystalline engineering thermoplastics which has been applied widely[1–3].However,the low notched impact strength of nylon 6 limits its application,and thus it is important to modify nylon 6[4].The most common method is blending the elastomer or plastic with nylon 6[5–7].The addition of the elastomer or plastic into nylon 6 is an effective method to improve the impact strength,but the effect is not very obvious due to poor compatibility.

Reactive microgel is an intramolecular crosslinked macromolecule in the size of 1 nm–1000 nm with different reactive groups such as carboxyl,hydroxyl,sulfonyl and amino groups on the surface or inside[8].This microgel can be crosslinked with other monomers or macromolecule under appropriate conditions to get the heterogeneous network polymer[9].Thus,blending reactive microgel with nylon 6 can improve the compatibility of the blends.It has an important theoretical significance and application prospects in improving the rheological and mechanical properties.As a combination of organic nano-particles and thermoplastic elastomer,reactive microgel is expected to provide a new idea for modifying nylon 6.

In this paper,the non-isothermal crystallization behavior of nylon 6 with varying content of reactive microgel was investigated by using differential scanning calorimetry(DSC).It is well known that properties of the crystalline polymer such as mechanical and thermal properties are strongly related to the crystallization behavior and crystal morphology of the materials.Therefore,to search for the best processing conditions in an industrial process and resulting products with better performance,it is necessary to study the crystallization behavior of crystalline polymer[10,11].Currently,there are many papers reporting methods for studying the non-isothermal crystallization kinetics of polymer,such as the Jeziorny theory[12],the Ozawa theory[13]and the Mo theory[14].Huang Zhaoge et al.[15]investigated the non-isothermal crystallization kinetics of nylon 6 by these three methods,and found that the Mo equation was more suitable to analyze the non-isothermal crystallization data comparing with the Jeziorny and the Ozawa theories.Thus,the Mo theory was used to study the non-isothermal crystallization kinetics of neat nylon 6 and reactive microgel/nylon 6 blends.The non-isothermal crystallization activation energies based on Kissinger's equation were also calculated.

2.Experimental

2.1.Materials

Nylon 6 was obtained from Hunan Yueyang Baling Petroleum&Chemical Co.,Ltd.The reactive microgel was self-prepared according to the method of Jana Machotová and Jaromír ?ňupárek[16].The reactive microgel was synthesized with the monomers of 2-hydroxyethyl methacrylate(HEMA),methyl methacrylate(MMA)and allyl methacrylate(AMA)using ammonium per sulfate as initiator by emulsion polymerization.The glass transition temperatures(Tg)of the reactive microgel was 122°C and the particle size in water phase of the reactive microgel was 110 nm.

2.2.Sample preparation

Nylon 6 was blended with reactive microgel(drying at 80°C under vacuum for two hours)in an internal mixer(type SU-70C,Changzhou Suyan Science and Technology Co.,Ltd.China).The mass ratios of reactive microgel to nylon 6 in the blends were 20%,30%,40%and 50%.The temperature of the barrel was set at 250°C and the rotational speed was 230 r·min?1.

2.3.DSC measurements

The non-isothermal crystallization kinetics was investigated by using a differential scanning calorimeter(type TG-DTA6300,Perkin Elmer Instrument Co.,Ltd.).All analyses were performed under a nitrogen atmosphere and the temperature scale of the DSC was calibrated with high purity(99.999%)indium metal.Sample with mass of 6–8 mg was heated at a rate of 20 °C·min?1from 25 °C to 250 °C and held there for 5 min to eliminate any previous thermal history,and then was cooled to the predetermined temperature(50°C)ata constant cooling rates of 10,15,20 and 25 °C·min?1,respectively.The nonisothermal crystallization curves were recorded as a function of time.

Table 1 Parameters of non-isothermal crystallization for nylon 6 and reactive microgel/nylon 6 blends

3.Results and Discussion

3.1.Non-isothermal crystallization behavior of nylon 6 and its blends

The non-isothermal crystallization curves of nylon 6 and its blends at various cooling rates are illustrated in Fig.1,and other corresponding parameters are listed in Table 1.In both cases,Tpand Tonsetshift towards lower temperature with increasing cooling rate.This phenomenon can be explained as follows:at the lower cooling rate,the nylon 6 molecular chains may have enough time to pack up in a unit cell and then their nuclei grows up at higher temperature;on the contrary,at higher cooling rate,the temperature may have gone down to a lower point before the nylon 6 molecular chains get ready to arrange regularly or to form crystal nuclei,resulting in a lower Tp[17].In addition,it is evident that the crystallization peak of blends is broadened as the cooling rate increases,which indicates that the range of crystallization temperature becomes wider.It is due to the fact that the crystallization time is shorter with increasing cooling rate,resulting in an harder orderly arrangement for the nylon 6 molecular chains.That is to say,fully crystallization is difficult in a short time,which leads to this phenomenon.

At the same cooling rate,Tonsetand Tpof the reactive microgel/nylon 6 blends shift towards lower temperature with the microgel content increases because the hydrogen bonds are formed between the amido groups of nylon 6 and the carboxyl groups of reactive microgel,which decrease the crystallization rate of nylon 6.

In the non-isothermal crystallization process,ΔT(Tonset–Tp)can reflect the crystallization rate,which means that the smaller the ΔT,the faster the crystallization rate[18].From Table 1,it is visible that for the same component,ΔT increases as the cooling rate increases,i.e.,the crystallization rate decreases.At the same cooling rate,ΔT increases with the increase of microgel content,i.e.,the crystallization rate decreases.That is to say,the crystallization of nylon 6 is hindered by reactive microgel.

It can be seen in Table 1 that for the same component,the crystallization half-time(t1/2)of the blends decreases with increasing cooling rate,i.e.,the crystallization rate increases.The main reason is that the molecular chain quickly gathers at low temperature in a short time with increasing cooling rate,which makes nucleation easier,and then the crystallization rate increases.On the other hand,for the same cooling rate,the t1/2of blends increases with increasing content of reactive microgel,indicating that the addition of reactive microgel reinforces the interactions between these blends,which limit the movement of molecular chain and reduce the diffusion capacity of the nylon 6,resulting in the decrease of the crystallization rate eventually.However,this conclusion does not conflict with the corresponding conclusions of ΔT because the two are analyzed from different perspectives.

The crystallization enthalpy(ΔHc)can reflect the relative crystallinity(Xt)of polymer.The greater the value of ΔHc,the larger the relative crystallinity of materials[19].It is obvious in Table 1 that the values of ΔHcof blends are bigger than those of neat nylon 6 with the addition of the reactive microgel,and ΔHcgets to the maximum when the content of reactive microgel is 30%.It means that adding reactive microgel can improve the relative crystallinity of nylon 6 to a certain degree,but when the content is too high,the forces between reactive microgel and nylon 6 enhance sharply,and then the relative crystallinity decreases instead.For the same component,ΔHcdecreases with increasing cooling rate,indicating that the increase of cooling rate makes the relative crystallinity of nylon 6 decrease.

3.2.Analysis based on the Mo theory

In order to better describe the non-isothermal crystallization process of reactive microgel/nylon 6 blends,Mo theory is applied,which points out that the relation between ? and t could be build up at a given relative crystallinity[20].The Mo equation is as follows:

and the Eq.(1)can be further rewritten as:

where the parameter F(T)=[K(T)/Zt]1/mrefers to the value of cooling rate,which is selected to reach a certain relative crystallinity in unit crystallization time.The value of F(T)can describe the crystallization speed of polymers under certain crystallization time.The smaller the value of F(T),the faster crystallization rate becomes.α is the ratio of the Avrami exponent n to the Ozawa exponent m(α=n/m)[21].

The plots of ln? versus ln t for nylon 6 and reactive microgel/nylon 6 blends at different relative crystallinity are shown in Fig.2.Obviously,ln? and ln t have a good linear relation,which means that the Mo theory can well describe the non-isothermal crystallization process.

The values of F(T)and α are achieved from the intercepts and the slopes in Fig.2,respectively,where are listed in Table 2.It can be seen that the values of α change lightly,while the values of F(T)increase with increasing the relative crystallinity,indicating that a faster cooling rate should be used to reach a given relative crystallinity in unit crystallization time.For the same relative crystallinity,the values of F(T)of blends are higher than those of neat nylon 6,indicating that the addition of reactive microgel can decrease the crystallization rate of nylon 6.

3.3.Crystallization activation energy

By taking into account the influence of various cooling rates ?,Kissinger[22]proposed that the activation energy could be determined by calculating the variation of the crystallization peak temperature with the cooling rate.The formula can be given as:

where ? is the cooling rate,R is the gas constant and Tpis the crystallization peak temperature.The values of In(?/Tp2)are plotted as function of 1000/Tpin Fig.3 and good linear relations are obtained.From the slopes of these lines generated from linear regression,the values of the activation energy(ΔE)are listed in Table 3.

Fig.2.Plots of lnt versus ln? for nylon 6(a),reactive microgel/nylon 6(20/80)(b),reactive microgel/nylon 6(30/70)(c),reactive microgel/nylon 6(40/60)(d)and reactive microgel/nylon 6(50/50)(e)blends at different relative crystallinities.

According to the definition of activation energy(ΔE),the larger the ΔE value,the weaker the molecular chain motion ability,and thus the crystallization rate is slower.As can be seen in Table 3,the value of ΔE of reactive microgel/nylon 6 blends is higher than that of neat nylon 6,which means that the reactive microgel can decrease the crystallization rate of nylon 6.The main reason may be the existence of strong hydrogen bonds between nylon 6 and reactive microgel,which limits the movement of molecular chain and reduces the diffusion capacity of the nylon 6.Thus,the crystallization activation energy is increased,leading to reduce the overall crystallization rate.However,the values of ΔE are not linearly increasing with the increase of reactive microgel.When the concentration of reactive microgel is 30%,the values of ΔE reach the maximum.It can explain the phenomenon in Table 1 that the crystallization enthalpy(ΔHc)achieves the maximum when the content of microgel reaches 30%.

4.Conclusions

The non-isothermal crystallization kinetics of neat nylon 6 and reactive microgel/nylon 6 blends were studied by DSC.The results show as follows:

(1)The crystallization onset temperature(Tonset)and crystallization peak temperature(Tp)decrease with the increase of the content of reactive microgel,while ΔT(Tonset–Tp),the crystallization halftime(t1/2)and the crystallization enthalpy(ΔHc)increase.Meanwhile,the relative crystallinity(Xt)of nylon 6 increases with increasing content of reactive microgel and Xtgets to the maximum when the content of reactive microgel is 30%.

(2)Mo equation is successful for analyzing the experimental data of non-isothermal crystallization kinetics.For the same relativecrystallinity,the values of F(T)of blends are higher than that of neat nylon 6,indicating that the addition of reactive microgel can decreases the crystallization rate of nylon 6.

Table 2 Values of F(T)and α from Mo equation for nylon 6 and reactive microgel/nylon 6 blends

Fig.3.–1000/T p curves for nylon 6 and reactive microgel/nylon 6 blends.

(3)The crystallization activation energy was evaluated by Kissinger equation.The values of ΔE of reactive microgel/nylon 6 blends are higher than that of neat nylon 6,which means that thereactive microgel can decrease the crystallization rate of nylon 6.However,the values of ΔE are not linearly increasing with the increase of reactive microgel.When the concentration of reactive microgel is 30%,the values of ΔE gets to the maximum.

Table 3 Activation energy of non-isothermal crystallization of nylon 6 and reactive microgel/nylon 6 blends