Question

3 Poly(3-hydroxybutanoic acid), a biodegradable polyester, is an insoluble, opaque material that is difficult to process into shapes. In contrast, the copolymer of 3 -hydroxybutanoic acid and 3-hydroxyoctanoic acid is a transparent polymer that shows good solubility in a number of organic solvents. Explain the difference in properties between these two polymers in terms of their structure.

Short answer

Answer: The difference in solubility and opacity between the two polymers is due to the presence of longer 3-hydroxyoctanoic acid monomers in the copolymer. This leads to increased flexibility and separation of chains, resulting in weaker intermolecular forces that improve solubility in organic solvents and better light transmission, giving a transparent appearance compared to the opaque homopolymer of poly(3-hydroxybutanoic acid).

Step-by-step solution

Identify the structure of poly(3-hydroxybutanoic acid)

The structure of poly(3-hydroxybutanoic acid) consists of repeating chains comprised of 3-hydroxybutanoic acid monomers. As each monomer within the chain is the same, this polymer is called homopolymer.

Identify the structure of the copolymer

The structure of the copolymer of 3-hydroxybutanoic acid and 3-hydroxyoctanoic acid is a chain of both 3-hydroxybutanoic acid and 3-hydroxyoctanoic acid monomers alternating or randomly dispersed throughout the chain. As the polymer contains more than one type of monomer, it is called a copolymer.

Compare the structural differences between the two polymers

While both poly(3-hydroxybutanoic acid) and the copolymer contain the 3-hydroxybutanoic acid monomers, the copolymer also includes the longer 3-hydroxyoctanoic acid monomers. These longer monomers in the copolymer form chains with increased separation and flexibility compared to the rigid structure of the homopolymer.

Explain the difference in properties in terms of molecular structure

The difference in molecular structure due to the presence of longer 3-hydroxyoctanoic acid monomers in the copolymer results in weaker intermolecular forces between polymer chains, which can easily slide past one another. This ultimately leads to improved solubility of the copolymer in organic solvents. Additionally, the increased flexibility and separation of chains in the copolymer allows for better light transmission, giving it a transparent appearance compared to the opaque homopolymer of poly(3-hydroxybutanoic acid).

Key concepts

Poly(3-hydroxybutanoic Acid)

Poly(3-hydroxybutanoic acid), commonly known by its abbreviation PHB, is a type of biodegradable polyester made from renewable resources. Its structure is composed of repeating units derived from 3-hydroxybutanoic acid, creating a long chain known as a homopolymer because it consists entirely of one type of monomer.

Due to hydrogen bonding and high crystallinity within these homopolymer chains, PHB is typically opaque and has a high melting point, which makes its processing more difficult. Additionally, this rigid structure of tightly packed molecules contributes to its insolubility in most solvents, limiting its range of application.

Copolymer Structure

Copolymers are polymers that are formed by linking two or more different types of monomers in contrast to the single monomer used in homopolymers. This varying monomer composition can result in a diversity of physical properties for the copolymer.

For instance, when considering the copolymer of 3-hydroxybutanoic acid and 3-hydroxyoctanoic acid, the inclusion of the latter introduces flexibility within the polymer chain. This is due to the longer carbon chains of the 3-hydroxyoctanoic acid which can lead to a more amorphous polymer structure, and thereby affect transparency, solubility, and other mechanical properties.

Intermolecular Forces in Polymers

Intermolecular forces, the attractions between molecules, play a significant role in determining a polymer's properties. Hydrogen bonding, van der Waals forces, and permanent dipole interactions are some of the intermolecular forces that affect how polymer chains interact with one another.

In polymers like PHB, strong hydrogen bonds cause the chains to be closely packed, resulting in a rigid and dense material. Copolymers with a mix of different monomer units may have less opportunity for strong, consistent intermolecular interactions, leading to decreased rigidity and increased solubility in solvents.

Homopolymer vs Copolymer

Homopolymers consist of one type of repeating monomer unit, while copolymers are made from at least two different types of monomers. The physical characteristics of these polymers can vary greatly due to the nature of their building blocks.

Homopolymers like PHB often display uniformity in properties such as strength and melting point but may lack the versatility required for certain applications. Copolymers, on the other hand, can be engineered to possess a balance of properties, such as enhanced flexibility and solubility like the copolymer of 3-hydroxybutanoic acid and 3-hydroxyoctanoic acid mentioned in the exercise.

Polymer Transparency

Polymer transparency is dependent on the ability of light to pass through a material without being scattered. A homopolymer such as PHB, with a highly crystalline and orderly structure, scatters light, resulting in an opaque appearance.

In contrast, the inclusion of different monomers in a copolymer can lead to a more irregular, less crystalline structure which allows light to transmit more easily, thus, making the copolymer transparent. This principle is evident in the transparency of the copolymer made from 3-hydroxybutanoic acid and 3-hydroxyoctanoic acid, where reduced crystallinity allows for light to pass through, rendering it transparent to the human eye.