Birefringent polymer having bimodal molecular weight
An optically anisotropic polymer thin film includes a crystallizable polymer and an additive configured to interact with the polymer (e.g., via π-π interactions) to facilitate chain alignment and, in some examples, create a higher crystalline content within the polymer thin film. The polymer thin film may be characterized by a bimodal molecular weight distribution where the molecular weight of the additive may be less than approximately 50% of the molecular weight of the crystallizable polymer. Example crystallizable polymers include polyethylene naphthalate, polyethylene terephthalate, polybutylene naphthalate, polybutylene terephthalate, as well as derivatives thereof. Example additives, which may occupy up to approximately 10 wt. % of the polymer thin film, include aromatic ester oligomers, aromatic amide oligomers, and polycyclic aromatic hydrocarbons, for example. The optically anisotropic polymer thin film may be characterized by a refractive index greater than approximately 1.7 and an in-plane birefringence greater than approximately 0.2.
1 . A method comprising:
forming a first polymer layer comprising a crystallizable polymer and an additive selected from the group consisting of an aromatic ester oligomer, an aromatic ester polymer, an aromatic amide oligomer, an aromatic amide polymer, and a polycyclic aromatic hydrocarbon, the first polymer layer having a bimodal molecular weight distribution;
forming a second polymer layer directly over the first polymer layer, the second polymer layer having a unimodal molecular weight distribution; and
producing an in-plane strain in the first polymer layer along a first direction in an amount sufficient to re-orient crystals or align polymer chains within the first polymer layer and form an optically anisotropic polymer layer.
2 . The method of claim 1 , wherein forming the first polymer layer comprises extruding a mixture containing the crystallizable polymer and the additive.
3 . The method of claim 1 , wherein producing the in-plane strain comprises applying a uniaxial stress to the first polymer layer.
4 . The method of claim 1 , wherein producing the in-plane strain comprises applying a biaxial stress to the first polymer layer.
5 . The method of claim 1 , wherein the crystallizable polymer is selected from the group consisting of polyethylene naphthalate, polyethylene terephthalate, polybutylene naphthalate, polybutylene terephthalate, and derivatives thereof.
6 . The method of claim 1 , wherein the aromatic ester oligomer is selected from the group consisting of polyethylene naphthalate, polyethylene terephthalate, polybutylene naphthalate, polybutylene terephthalate, and derivatives thereof.
7 . The method of claim 1 , wherein the aromatic amide oligomer is terminated by a non-reactive group.
8 . The method of claim 1 , wherein the additive comprises an acene selected from the group consisting of anthracene, tetracene, pentacene, and derivatives thereof.
9 . The method of claim 1 , wherein the additive comprises less than approximately 80 wt. % of the first polymer layer.
10 . The method of claim 1 , wherein a molecular weight of the additive is less than approximately 70% of a molecular weight of the crystallizable polymer.
11 . The method of claim 1 , wherein the optically anisotropic polymer layer has a refractive index greater than approximately 1.7 and an in-plane birefringence greater than approximately 0.2.
12 . The method of claim 1 , wherein the optically anisotropic polymer layer has a transmissivity within a visible light spectrum of at least approximately 20% and less than approximately 10% bulk haze.
13 . A method comprising:
forming a first polymer layer comprising a crystallizable polymer and an additive selected from the group consisting of an aromatic ester oligomer, an aromatic ester polymer, an aromatic amide oligomer, an aromatic amide polymer, and a polycyclic aromatic hydrocarbon, the first polymer layer having a bimodal molecular weight distribution;
forming a second polymer layer directly over the first polymer layer, the second polymer layer having a unimodal molecular weight distribution; and
producing an in-plane strain in the first polymer layer along a first direction in an amount sufficient to re-orient crystals or align polymer chains within the first polymer layer and form an optically anisotropic polymer layer, wherein the optically anisotropic polymer layer has a transmissivity within a visible light spectrum of at least approximately 20%, a refractive index greater than approximately 1.7, and an in-plane birefringence greater than approximately 0.2.
14 . The method of claim 1 , wherein the second polymer layer comprises an amorphous polymer.
15 . The method of claim 1 , wherein the second polymer layer comprises an amorphous crystallizable polymer.
16 . The method of claim 1 , further comprising co-extruding the second polymer layer and the first polymer layer to form a multilayer polymer thin film.
17 . The method of claim 16 , wherein the multilayer polymer thin film comprises alternating layers of the first polymer layer and the second polymer layer.