Mechanisms Of Crystallization And The Form Ⅱ To Ⅰ Transition In Polybutene-1

Polybutene-1 is one kind of poly-α-olefins and four modifications of it have been found.Polybutene-1 products have excellent mechanical properties and long service life,especially its creep resistance at high temperatures is significantly better than the other poly-α-olefin products.However,it also has a feature,that is,the solid-solid transformation.In general,when the products of polybutene homopolymers are solidified from the melt,there is only kinetically favored form Ⅱ in the as-crystallized products.But form Ⅱ is metastable in thermodynamics and it will spontaneously transform into the thermodynamically stable form Ⅰ after annealing at room temperature and atmospheric pressure.The transition process can basically take several weeks,which prolongs the production period and increases the production cost.Besides,the price of polybutene raw materials is relatively high,then it is hard to compete with polyethylene and polypropylene in commercial development.Even so,the Polybutene-1 tube can effectively bear the internal pressure for a long time and the creep resistance of it is extraordinary.Therefore,Polybutene-1 is named as "plastic gold",which indicates that its application value has been generally recognized.In this dissertation,three kinds of isotactic Polybutene-1(iPB-1)homopolymers were studied systematically and deeply.By the means of differential scanning calorimeter(DSC),polarization microscope(POM)and wide-angle X-ray diffraction(WAXD),the melt crystallization,cold crystallization and the phase transition from form Ⅱ to Ⅰ of i PB-1 have been investigated.The main research contents are as follows:1.The melt crystallization of iPB-1In this chapter,the melt crystallization properties of two iPB-1 samples were studied by DSC.Firstly,a special melt memory effect beyond the equilibrium melting point was found in the commercial iPB-1 sample PB 0400.After the purification,the melt memory effect of the purified sample disappeared completely at the temperature close to the equilibrium melting point,which indicated that the effect probably came from the influence of additives rather than the property of iPB-1 itself.At the same time,it can be observed via POM that the primary nucleation density of PB 0400 increased,while the growth rate was not affected at all.Therefore,the memory effect promoted iPB-1 to overcome the primary nucleation barrier,so it only affects the primary nucleation process.Then,we investigated the melt crystallization and transformation of the low molar mass sample PB-L which was synthesized in the laboratory.We found that PB-L could only form form Ⅱ when crystallized from the homogeneous melt,but it could get formⅠ’ under the effect of self-nucleation/seeding.The transformation from form Ⅱ to Ⅰ of PB-L was very fast so that there were always nuclei of form Ⅰ remaining in the melt at the self-seeding temperature.When the sample was heated to 110℃,the nuclei of formⅡ crystals were destroyed while the nuclei of form Ⅰ crystals could still be retained.With the help of these nuclei/seeds,form Ⅰ’ can complete with form Ⅱ during isothermal crystallization.The higher the content of form Ⅰ in the initial sample,the more favorable the crystallization of form Ⅰ’.what’s more,the crystallization of form Ⅱ could be suppressed by the self-seeding of form Ⅰ.In this experiment,it was found that the crystallization of form Ⅱ could not be accelerated by heterogeneous nucleation on the form Ⅰ seeds.It is explained that the crystallization of form Ⅱ of PB-L is restricted since its size of critical nuclei is much bigger than that of form Ⅰ’.2.The cold crystallization of iPB-1In this chapter,the cold crystallization behavior of iPB-1 was investigated.The homogeneous melt was quenched to a temperature below Tg and the amorphous was relaxed for different times before isothermal crystallization.The influence of relaxation on cold crystallization is explored.First of all,we tested the cold crystallization of the purified sample and found that with the increase of relaxation time ta,the gross crystallization rate of cold crystallization was accelerated.Through POM,it was proved that the acceleration mainly came from the increased primary nuclei in the cold crystallization system.The enthalpy relaxation of the purified iPB-1 sample at-30℃only took 2 s,and the enthalpy of the non-equilibrium amorphous basically reached the equilibrium state after the enthalpy relaxation.After the enthalpy relaxation,the noncooperative local motions of chains in the system gradually lead to crystal nuclei.If the relaxation time is too short,the nuclei formed at-30℃ are unstable and cannot be used as primary nuclei to accelerate the crystallization.However,these unstable nuclei are easy to develop into the stable ones during the heating process to the crystallization temperature,and the fast heating rate(usually at least more than 10000 K/s)can inhibit the nucleation during heating.If the relaxation time is long enough,the stable nuclei can form at-30℃,and the number of these nuclei has a saturation value.At the same time,these nuclei show size distribution.When crystallizing at a low temperature,the size of critical nuclei is small so that the unstable nuclei formed in a short relaxation time can act as primary nuclei to promote crystallization and the number of primary nuclei is not affected by the heating rate.When crystallizing at a high temperature,the required size of critical nuclei is large so that the number of effective nuclei formed during relaxation is less,then the development of nuclei during heating has a more obvious effect on the number of primary nuclei with a high crystallization temperature.Moreover,the cold crystallization of PB-L showed that two kinds of nuclei affected the crystallization.Among them,the first ones formed in a short relaxation time did not need complete enthalpy relaxation,and the number reaches the maximum in about 2 s;while the second one formed only after the completion of enthalpy relaxation,and the number reaches the maximum in about 100 s.Compared with the results of the purified sample,it was found that the nucleation needing more ta should come from homogeneous nucleation,while the nuclei formed faster is from heterogeneous nucleation by the adsorption of impurities on the molecular chain.The thermal stability of the heterogeneous nuclei is worse than that of the homogeneous ones.The desorbing occurs at about 80℃,where the crystal nucleus can be destroyed,and the heterogeneous nuclei need 100℃ to be completely destroyed.3.The phase transition from form Ⅱ to Ⅰ of iPB-1 with low molar massIn this chapter,the phase transformation behavior of PB-L was investigated.There are few tether molecules in melt crystallized PB-L samples.When cooled to a lower temperature for annealing,the unbalanced shrinkage between the crystalline and amorphous regions makes the tie chains the stress concentration points,which can effectively promote the formation of crystal Ⅰ nucleus.It is found that even annealed at a temperature lower than Tg,the PB-L sample gradually transformed from form Ⅱ to I.In the thicker lamellae,there are fewer tie molecules and the chains in form Ⅱ crystals are limited by the frozen molecular chain in the amorphous region.The transition barrier is relatively low so that the transformation in the thick lamellae of form Ⅱ can continue below Tg.Using the stepwise annealing method,the PB-L sample is annealed at two temperatures.The nuclei formed during the low-temperature annealing can play a full role during the high-temperature annealing.When annealed at high temperature,the transition barrier for the thick lamellae is higher than that of the thin lamellae,and the number of form Ⅰ nuclei in the thick lamellae is less.Therefore,the transformation advantage of the thin lamellae is more obvious,where form Ⅱ rapidly transformed into form Ⅰ through thermal fluctuation.The melting point of the form Ⅰ transformed from the thin lamellae is lower than that of the form Ⅰ from the thick lamellae when annealed at low temperature.Therefore,the PB-L sample shows two kinds of form Ⅰ crystals with different melting points after stepwise annealing.Form Ⅰ’ can grow in the amorphous during high-temperature annealing and the fast transformed form Ⅰ can play the role of athermal nucleation.The crystallization temperature of the initial sample decides the growth environment of form Ⅰ’,while the annealing temperature decides the melting point of form Ⅰ’.Once the form Ⅰ’ is formed in the amorphous region,the motion ability of the chains in the original amorphous region is further limited.Therefore,the sample with a higher Tc can grow form Ⅰ’crystals easier but the crystal transformation rate is slower.