Crystallization in confined environments usually induces polymers showing complicated crystallization kinetics and unusual crystalline structure. Beyond the typical confined polymer systems, pre‐existing crystals can also exert confinement effects on the subsequent crystallization of polymorphic or multi‐component polymers; this, however, is not well understood at present. Herein, poly(l‐lactic acid)/poly(d‐lactic acid) (PLLA/PDLA, abbreviated as L/D) asymmetric blends with various PDLA fractions (f_D = 0.02–0.5) are chosen as a model system and the effects of pre‐existing stereocomplexes (SCs) on the crystallization kinetics and polymorphic structure are investigated. It is found that unusual β‐form homocrystals (HCs) of poly(lactic acid) can be formed in an asymmetric L/D blend, which are strongly influenced by the molecular weights (MWs) of the used polymers, L/D mixing ratio, thermal treatment temperature (T_max) and crystallization temperature (T_c). The formation of β‐HCs is preferred in asymmetric L/D blends with low and medium MWs, medium f_D (0.1–0.2), medium T_max (170–200°C), and low T_c (70–110°C). The metastable β‐HCs reorganize into the more stable α‐HCs via melt recrystallization in the heating process. It is proposed that the β‐HC formation stems from the constraining effects of pre‐existing SCs; this constraining effect is governed by the content of pre‐existing unmelted SCs in the thermally treated samples.

中文翻译：

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预先存在的立体配合物的约束效应引起的聚（l-乳酸）/聚（d-乳酸）不对称共混物中异常均晶的可控形成

在密闭环境中结晶通常会诱导聚合物显示复杂的结晶动力学和异常的晶体结构。除了典型的受限聚合物体系外，预先存在的晶体还可以对多晶型或多组分聚合物的后续结晶产生约束作用。但是，目前尚不十分清楚。在此，聚（l-乳酸）/聚（d-乳酸）（PLLA / PDLA，缩写为L / D）与各种PDLA组分（f _D= 0.02–0.5）作为模型系统，并研究了预先存在的立体配合物（SCs）对结晶动力学和多晶型结构的影响。发现在不对称的L / D共混物中会形成不寻常的聚乳酸β形均晶（HCs），这受所用聚合物的分子量（MWs），L / D混合比的强烈影响，热处理温度（T _max）和结晶温度（T _c）。在低和中等MW，中等f _D（0.1-0.2），中等T _max（170-200°C）和低T _c的不对称L / D共混物中，优选形成β-HCs（70–110°C）。在加热过程中，亚稳态β-HCs通过熔融重结晶重组为更稳定的α-HCs。有人认为β-HC的形成是由于先前SC的抑制作用引起的。这种约束效果受热处理样品中预先存在的未熔融SC含量的控制。