Introduction to the characteristics of the hottest

Introduction to the characteristics and application of polycaprolactone (Part 1)

Abstract: through the introduction of the biodegradability and shape temperature control memory mechanism of polycaprolactone (PCL), this paper analyzes the molding and processing technology of some polycaprolactone polymer materials at present, and introduces the interventional use of this kind of products in medical, daily use, agriculture and other industries. Finally, the application prospect of constructing polycaprolactone new materials by using special processing technology or misoperation technology is prospected

key words: polycaprolactone degradability memory process

since the 1960s, PCL Polycaprolacton has attracted extensive attention because of its superior biodegradability and memory, and its related research has also developed rapidly. PCL is a semi crystalline polymer with melting point. The selling price of this product (stroller) is 59 ~ 64 ℃, and the glass transition temperature is -60 ℃. There are five nonpolar methylene CH2 and one polar ester coo on its structural repeating unit, namely - (coo-ch2ch2ch2ch2ch2ch2 -) P n. such a structure makes PCL have good flexibility and processability, and this material has good biocompatibility. This structural feature, on the one hand, makes it have shape memory. After deformation and fixation, the products with initial shape can be restored to their original shape by heating and other external condition stimulation. On the other hand, the material can be blended with starch and other substances to produce fully biodegradable materials. At present, the characteristics of these two aspects have been applied in many fields, especially in the medical field, such as tape, bandage, orthotics, suture, drug slow-release agents and so on. 1. Characteristic mechanism of polycaprolactone

1.1 degradation mechanism

polycaprolactone is a biodegradable polymer material synthesized by chemistry. Its molecular structure introduces the ester group structure coo. In nature, the ester group structure is easy to be decomposed by microorganisms or enzymes, and the final products are CO2 and H2O, [1]: the specific process is as follows

first stage: hydration. The material absorbs water from the surrounding environment, which takes several days or months, depending on the performance and surface area of the material

the second stage: the chemical chain of the polymer backbone is broken due to hydrolysis or enzymatic hydrolysis, resulting in the decline of molecular weight and mechanical properties

the third stage: after the strength loss, the polymer becomes oligomer fragments, and the overall mass begins to decrease

stage 4: oligomers are further hydrolyzed into smaller fragments, which are absorbed by phagocytes, or further hydrolyzed to produce CO2 and H2O

according to relevant materials, PCL products with a molecular weight of 30000 disappear in the soil after one year, so PCL is recommended as an "environment-friendly" packaging material

1.2 mechanism of shape memory

the shape memory function of polymer polycaprolactone (PCL) materials mainly comes from the incompletely compatible two phases in the material: the fixed phase that maintains the shape of the molded product and the reversible phase that softens and hardens reversibly with temperature change. Reversible phases, such as crystalline state with low melting point (TM) or glassy state with low glass transition temperature (TG), have physical crosslinking structure. The stationary phase can have physical cross-linking structure (such as molecular entanglement formed by the phase with higher TM or TG at lower temperature) or chemical cross-linking structure

the stationary phase and the reversible phase have different softening temperatures (identified as Ta and TB respectively). In the one-time molding process, the material is heated to above TA, at this time, both the stationary phase and the reversible phase are in a softening state, and they are cooled below TB after molding. The stationary phase and the reversible phase are hardened successively, and the material is formed. The secondary molding is to heat the molding material to the softening temperature of the reversible phase (TB t TA). The reversible phase softening can be made into any second shape, and the new shape can be obtained by maintaining the stress and cooling and fixing. If the reversible phase is softened by heating to an appropriate temperature again, the stationary phase will restore the product to its original shape under the action of restoring stress. The basic principle of this shape memory effect [2,3] can be simply expressed as the cycle of "memory initial state → fixed change state → restore initial state" [4], that is,

in the formula, L - the original length of the sample; △ L-shaped variable. At present, Yamaguchi of Japan has put forward a more appropriate mechanical model explanation for PCL shape memory materials. He believes that [6] shape memory materials in rubber colloidal state are under external force σ The deformation rate under the action includes three parts, namely, instantaneous deformation rate ε 0. Relaxation deformation rate ε R and creep deformation rate ε c： ε=ε 0+ ε r+ ε C

after the external force is released, the solidified deformation state will recover instantly when heated ε C 'and creep deformation rate recovery ε c′。 Deformation rate due to relaxation ε R is plastic deformation, and its deformation rate recovers ε R 'is extremely small, but when the heating reaches the shape recovery temperature, ε R

deformation rate recovery occurs ε n′。 To the introduction of investment and financing of carbon fiber and its composites, the material has been restored to its original state macroscopically

2. Modification of polycaprolactone [7]

polycaprolactone (PCL) is a product developed by dicell chemical industry company. The molecular weight of the material is 40000, the melting point is 59 ~ 64 ℃, the glass transition temperature is -60 ℃, the orthopedic temperature is 60 ~ 70 ℃, and the decomposition temperature is 200 ℃. It can be seen that its melting point and decomposition temperature are relatively low, so it is not suitable for general plastic molding process. In addition, the flexibility of its structure, as an orthopedic support, is not strong enough. Therefore, it is necessary to modify PCL. Through the modification of PCL, various new polycaprolactone materials that can meet the existing (or improved processing requirements and achieve specific functional requirements) can be constructed. At present, the main modification methods are ① pcl/tdl/bdo polyurethane system, which uses hydroxyl terminated polycaprolactone (PCL), 2,4-toluene diisocyanate (2,4-TDI), chain extender 1,4-butanediol (BDO) as raw materials, and adopts two-step solution polymerization to prepare multi block polyurethane samples, in which PCL is the soft segment and tdi-bdo is the hard segment; ② In pcl/mdi/bdo system, diphenylmethane diisocyanate (MDI) is chain extended, PCL is soft segment, and mdi/bdo is hard segment. ③ Pcl/isopropylidene acetone (IPDI) 12 hydroxydimethylphenyl acrylate system