Understanding the PHA Family: From Molecular Structure to PHB, PHBV, PHBH and P34HB

PHA is often spoken about as if it were one material.

In reality, PolyHydroxyAlkanoates (PHAs) are an entire family of polyesters. Small changes in their molecular structure can lead to major differences in stiffness, flexibility, crystallinity, melting behaviour, processing and ultimately the applications in which they can be used.

This is why simply asking for “PHA” is rarely enough when developing a product.

A rigid injection-moulded part, a flexible film and an elastomeric component may all be made from polymers that belong to the PHA family, while behaving very differently.

To understand why, we need to start at the molecular level.


Crystallinity is strongly influenced by comonomer content.
The labels above describe general trends only. In copolymers such as PHBV, PHBH and P34HB, changing the amount of 3HV, 3HHx or 4HB can significantly affect how efficiently the polymer chains pack together, and therefore how crystalline or amorphous the material becomes.

Helian Polymers supplies PHB, PHBV, PHBH and P34HB. Each type has a different molecular structure and, depending on its comonomer content, can show a different balance of crystallinity, stiffness, toughness, flexibility and processing behaviour.

PHA is a family of polyesters

PHA stands for PolyHydroxyAlkanoate.

These polymers are naturally produced by microorganisms, which can accumulate PHA inside their cells as a carbon and energy reserve. Chemically, PHAs are polyesters. Their polymer backbone contains repeating ester bonds, but the exact molecular structure of the repeating units can vary.

That variation is what creates the large PHA family.

More than a hundred different PHA monomer structures have been identified, although only a smaller number are currently of major commercial and technological interest. Frequently discussed monomers include 3-hydroxybutyrate, 3-hydroxyvalerate, 3-hydroxyhexanoate, 4-hydroxybutyrate and various longer-chain hydroxyalkanoates. Source: Jaffur et al., International Journal of Biological Macromolecules, 2025

First: what are monomers and polymers?

A monomer is a relatively small molecular building block. A polymer consists of many repeating units of these monomers connected together into a long chain.

For PHA, these first molecular steps happen inside microorganisms. During fermentation, bacteria convert renewable feedstocks into PHA and build the polymer inside their cells.

A key enzyme in this process is PHA synthase. It links activated hydroxyalkanoate building blocks together, forming the growing PHA polymer chain inside the cell. The PHA is then stored by the microorganism as intracellular granules, which can later be recovered and purified.

A simple way to imagine this is:

Feedstock → bacteria → monomer building blocks → repeat units → polymer chain → PHA granules inside the cell → PHA purified powder 

After fermentation, the PHA is extracted from the cells and purified in multiple steps as a powder. After that, PHA powder can be processed into pellets and eventually converted into final products.

This is fundamentally different from many conventional plastics, where the molecular building blocks are typically produced through petrochemical processes of turning crude oil into monomers for synthetic polymers.

For PHA, changing the monomers and their arrangement changes the molecular structure of the polymer, which in turn can change properties such as crystallinity, flexibility, melting behaviour and processing.

What does the general PHA structure look like?

Most PHAs share a similar polyester backbone, but the group attached to that backbone can vary and the length of the backbone can very as well. In simplified form, the PHA repeat unit can be represented as:

–O–CH(R)–(CH2)m–CO–

The R-group, or side chain, is important because its size and structure will influence how neighbouring polymer chains can arrange themselves. If polymer chains can pack together very regularly, they can form ordered crystalline regions. If their structure makes regular packing more difficult, a larger fraction may remain amorphous.

That molecular difference eventually becomes visible at the macroscopic level in properties such as stiffness, flexibility, toughness and thermal behaviour.

What do 3HB, 3HV, 3HHx and 4HB mean?

A lot of PHA terminology initially looks more complicated than it actually is.

Some of the most important monomers are:

3HB — 3 HydroxyButyric acid
A four-carbon hydroxyalkanoate and the building block of PHB.

3HV — 3-HydroxyValeric acid
A five-carbon hydroxyalkanoate, commonly incorporated together with 3HB to form P(3HB-co-3HV), often shortened to PHBV.

3HHx — 3-HydroxyHeXanoic acid
A six-carbon hydroxyalkanoate used together with 3HB  and forms P(3HB-co-3HHx), commonly referred as PHBH or PHBHHx.

4HB — 4-HydroxyButyric acid
Also contains four carbon atoms, but the hydroxyl functionality is positioned differently from 3HB. When combined with 3HB, it forms P(3HB-co-4HB), often shortened to P34HB.

The number therefore refers to the position of the hydroxy group in the hydroxyalkanoate, while names such as butyrate, valerate and hexanoate relate to the carbon structure of the monomer. These differences may appear small on paper, but inside a polymer containing thousands of repeat units, they can dramatically change material behaviour.

Homopolymers vs copolymers

Another important distinction is the difference between a homopolymer and a copolymer. A homopolymer consists primarily of one type of repeating monomer unit.

PHB, for example, is:

P(3HB) = poly(3-hydroxybutyrate)

A copolymer contains two or more different monomer units.

Examples include:

PHBV = P(3HB-co-3HV)

PHBH = P(3HB-co-3HHx)

P34HB = P(3HB-co-4HB)

The second monomer does more than simply change the name.

It can disrupt how easily the chains arrange themselves into ordered structures and can therefore change the crystallinity, melting behaviour and mechanical response of the polymer. Research on PHBV and P34HB, for example, shows that increasing co-monomer content can substantially reduce melting temperature and crystallinity. Source: Huang et al., Molecules, 2026.

PHB: the simplest starting point

PHB, or poly(3-hydroxybutyrate), is one of the best-known members of the PHA family.

Its polymer chain consists of repeating 3HB units.
Because these units have a relatively regular molecular structure, PHB chains can arrange themselves into crystalline regions. This contributes to PHB generally being a relatively strong and highly crystalline material.

However, this high crystallinity is also associated with one of PHB's main challenges: it can be brittle and has relatively low elongation compared with more flexible PHA copolymers. Source: Atarés et al., Foods, 2024.

PHB also has a comparatively narrow thermal processing window because its melting temperature can be relatively close to temperatures at which thermal degradation becomes significant. 

Also, PHB after processing undergoes a post crystallization process under normal storage conditions. This process increases crystallinity and as a result embrittlement of the material. Sourse: Bioengineering 2023, 10, 1382

Typical melting points mentioned in the literature vary by molecular weight, thermal history and measurement method, but PHB is often reported with melting temperatures around 175 °C range and relatively high crystallinity. Source: Naser et al., RSC Advances, 2021.

That makes PHB an excellent example of an important principle:

Source: Jon Donkers

PHBV: what happens when 3HV is introduced?

PHBV is a copolymer containing both 3HB and 3HV units. Adding 3HV changes the regularity of the PHB chain. The extra carbon associated with the 3HV unit alters the molecular structure enough to interfere with the highly regular packing found in PHB.

As the 3HV fraction changes, properties such as crystallinity, melting temperature, toughness and flexibility can also change.

Studies consistently show that increasing 3HV content tends to reduce melting temperature and crystallinity compared with PHB and can make the material tougher and more flexible. Source: Atarés et al., Foods, 2024.

This means that PHBV is not one fixed material. A PHBV containing a few mol% 3HV can behave very differently from a PHBV containing a much larger fraction of 3HV.

The name PHBV therefore tells you the polymer family, but not yet the complete performance profile.


Source: Jon Donkers

PHBH: introducing 3HHx

PHBH is commonly used as shorthand for P(3HB-co-3HHx).

Here, 3HB is combined with 3-hydroxyhexanoate.

Because 3HHx contains a larger side chain than 3HB and 3HV, introducing it into the polymer changes how easily neighbouring chains can pack together.

As the 3HHx content increases, PHBH materials can become less crystalline, softer and considerably more ductile than PHB.

Published data on P(3HB-co-3HHx) show lower melting temperatures and much greater elongation at break than PHB, although actual properties depend strongly on the 3HHx fraction and its distribution along the polymer chain. Source: Boey et al., Polymers, 2021

This is one of the clearest examples of how a relatively small change at the monomer level can create a very different engineering material.

PHBH can therefore cover properties far removed from the rigid and brittle behaviour commonly associated with pure PHB.

Source: Jon Donkers

A practical PHBH example

A good example of how co-monomer content changes material behaviour can be seen in the Bluepha BP330 and BP350 grades.

BP330 contains roughly 6% 3HHx co-monomer, while BP350 contains around 10% 3HHx. The lower 3HHx content of BP330 is associated with a higher degree of crystallinity, while BP350 is generally less crystalline and more flexible.

This difference can influence molecular behaviour, processing parameters and mechanical performance, which means the two grades may be suitable for different applications even though both belong to the PHBH family.

P34HB: same carbon count, different structure

P34HB is another important PHA copolymer.

Its full name is:

poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

At first glance, 3HB and 4HB may appear very similar because both contain four carbon atoms.

But the location of the hydroxy group is different.

This changes the resulting polymer repeat unit.

As the proportion of 4HB increases, the molecular regularity and crystallisation behaviour of the material can change substantially and often leave the material overall amorphous (>30% 4HB). This is therefore associated with high flexibility material.

Published datasets show that increasing 4HB content can strongly reduce melting temperature and crystallinity and increase elongation. Some high-4HB compositions behave far more like flexible thermoplastics than conventional PHB. Source: Huang et al., Molecules, 2026.

This makes P34HB another good reminder that:

Source: Jon Donkers

Why the monomer ratio matters

Knowing that a material is PHBV, PHBH or P34HB is only the starting point. The amount of each co-monomer also matters.

For example, a PHBV with 5 mol% 3HV can behave very differently from one with a co-monomer content of 20 mol% 3HV . The same principle applies to 3HHx in PHBH and 4HB in P34HB.

Changing the co-monomer content can significantly affect melting temperature, crystallinity and mechanical behaviour . Source: Huang et al., Molecules, 2026.

What is crystallinity?

Most thermoplastic polymers are neither completely crystalline nor completely amorphous. Instead, they usually contain a combination of both.

Crystalline regions are areas where polymer chains are arranged in a relatively ordered structure.

Amorphous regions are less ordered, with polymer chains arranged more randomly.

A more common form for polymer materials is known as semi-crystalline. The proportion of crystalline regions is referred to as the degree of crystallinity .

Crystallinity can influence important material properties such as stiffness, dimensional stability, barrier behaviour, transparency, toughness, shrinkage, crystallisation time, processing behaviour and biodegradation behaviour.

For PHA, crystallinity is especially important because relatively small changes in monomer composition can strongly affect how easily the polymer chains arrange themselves into ordered structures. PHB is often relatively crystalline, while the incorporation of co-monomers such as 3HV, 3HHx or 4HB can disrupt chain regularity and change crystallisation behaviour. Source: Huang et al., Molecules, 2026. 

Short-chain-length and medium-chain-length PHA

PHAs are also commonly divided according to the number of carbon atoms in their monomer units.

A commonly used classification is:

scl-PHA — short-chain-length PHA
Approximately 3–5 carbon atoms per monomer.

Examples include PHB and PHBV.

mcl-PHA — medium-chain-length PHA
Approximately 6–14 carbon atoms per monomer.

These often contain larger side chains and tend to behave more like flexible or elastomeric polymers.

lcl-PHA — long-chain-length PHA
More than approximately 14 carbon atoms per monomer, although these are much less common commercially.

In general, scl-PHAs tend to show higher crystallinity and stiffness, while many mcl-PHAs are more amorphous, flexible and elastic. The exact behaviour still depends on molecular composition and polymer architecture. Source: Dey et al., International Journal of Biological Macromolecules, 2024.

PHBH is particularly interesting because it combines a short-chain 3HB unit with the longer 3HHx unit, allowing its properties to be tuned through copolymer composition.

Tg, Tc and Tm: three temperatures that tell us a lot

Three important thermal properties of PHA are the glass transition temperature (Tg), crystallisation temperature (Tc) and melting temperature (Tm).

Glass transition temperature — Tg
Tg mainly relates to the amorphous part of the polymer. Below Tg, molecular movement is more restricted. Above Tg, the chains gain more mobility, which can make the material softer or more flexible.

Crystallisation temperature — Tc
Tc is the temperature at which polymer chains organise into crystalline regions during cooling. This is especially important for PHA because crystallisation behaviour can strongly influence cooling time, shrinkage, dimensional stability and final stiffness.

Melting temperature — Tm
Tm relates to the crystalline regions. At this temperature, the ordered crystalline structures melt and the polymer can flow.

For semicrystalline PHAs, Tg, Tc and Tm all matter because together they influence material behaviour, processing conditions and final product performance. Different PHA compositions can therefore show very different thermal behaviour. Source: Naser et al., RSC Advances, 2021.

Why crystallisation rate matters during processing

Degree of crystallinity is only part of the story. How quickly a PHA crystallizes is just as important. 

The rate of crystallization is an important factor, as this determines many processing parameters. During processing, the polymer is melted and then cooled. As it cools, the chains can reorganize into crystalline regions. The rate in which this happens determines the processability of a specific PHA, and is thus also related to the molecular background of PHA.

Due to the low Tg of PHA, the molecular mobility could allow the molecules to move into their packed state after processing. This is called post-crystallization. If this process transpires after the product leaves the mould or cooling system, its dimensions and properties can still change. 

Crystallization behaviour therefore influences cooling time, cycle time, shrinkage, warpage, dimensional stability and final stiffness.

If crystallisation under specific processing conditions is not optimal, an additive in the form of a nucleating agent can be introduced.

Molecular weight matters too

Even when two PHA materials contain the same monomers in the same ratio, they can still behave differently because of molecular weight.

In simple terms, molecular weight relates to the length of the polymer chains. Longer chains generally create more entanglement, which can influence melt strength and mechanical properties.

Polymer engineers therefore look at both molecular weight and its distribution.

Processing can also change molecular weight. PHA is very sensitive to heat treatment. Excessive heat or long residence times can damage PHA chains, which can affect processing behaviour and final material performance.

From molecular structure to mechanical properties

Molecular and thermal properties ultimately determine how a PHA behaves in practice.

Higher crystallinity often means more stiffness, but usually less ductility.
Lower Tg allows more chain mobility and can increase flexibility.
Co-monomer content can reduce chain regularity and change toughness or flexibility.
Higher molecular weight can increase chain entanglement and influence melt strength.

These effects interact with each other, which is why simple statements such as “PHB is rigid” or “PHBH is flexible” are only generalisations.

The behaviour of a real PHA grade depends on its complete molecular structure, composition and formulation.

The polymer is only the starting point

Commercial PHA materials are often formulations or compounds, not just the base polymer.

They may contain other polymers, plasticisers, nucleating agents, fillers, fibres, pigments, stabilisers or processing aids to adjust processing and final performance.

This is where compounding becomes important: different components are mixed under controlled melt-processing conditions to create a material tailored to a specific application.

So choosing the right PHA is only the first step. The final formulation can be just as important.

Why two materials labelled “PHA” can behave completely differently

Two materials can both be called PHA and still behave very differently.

That is because material performance depends on much more than the polymer family alone. Important factors include the monomer type and ratio, molecular weight, crystallinity, Tg and Tm, additives, formulation and processing history.

Together, these factors influence properties such as stiffness, flexibility, melt behaviour, shrinkage and final product performance.

This is why selecting a PHA based only on the name of the polymer is rarely enough.

From molecular structure to processing

Molecular structure directly affects how a PHA behaves during processing.

Depending on the grade and formulation, it can influence processing temperature, melt viscosity, cooling time, crystallisation, shrinkage, warpage and final mechanical properties.

PHB is a good example: its melting temperature can lie relatively close to the temperature range where thermal degradation becomes relevant, which can make processing more demanding. Copolymerisation can lower the melting temperature and change processability.

However, successful processing still depends on more than the polymer itself. Machine settings, cooling, product geometry and formulation all matter. Source: Atarés et al., Foods, 2024.

Does PHA type also affect biodegradation?

Yes. Polymer structure and crystallinity can influence how easily microorganisms and enzymes access the material. More crystalline regions are often less accessible than amorphous regions.

However, biodegradation depends on much more than polymer chemistry alone. Product thickness, surface area, formulation and environmental conditions also play an important role.

For a deeper explanation, read our PHA Biodegradability Explained article.

How do you choose the right PHA?

The best starting point is not “Which PHA is best?” but “What does the final product need to do?”

Important questions include whether the product needs to be rigid or flexible, impact resistant, transparent, injection moulded, extruded or blown into film, as well as its use temperature, crystallisation behaviour, thickness and intended end-of-life route.

Once these requirements are clear, it becomes much easier to select the right PHA type, monomer composition, molecular weight and formulation for the application.

How Helian Polymers works with the PHA family

At Helian Polymers, material selection starts with the application.

Because PHA is a broad polymer family, different building blocks can be evaluated depending on the required mechanical properties, processing route, crystallisation behaviour and end-use conditions.

Helian works closely with specialised PHA producers including TianAn Biopolymer, CJ Biomaterials, PhaBuilder and Bluepha, giving access to different polymer families such as PHB, PHBV, PHBH and P34HB.

When an existing grade does not provide the required balance of properties, the material can be further optimised through formulation and compounding. This allows properties such as stiffness, flexibility, processing behaviour and crystallisation to be adjusted toward the requirements of the final product.

At Helian Polymers, we can cover the majority of conventional fossil-based polymers mechanical properties by blending different types of PHA.

The development process therefore typically moves from:

Application requirements → polymer selection → formulation → compounding → processing trials → testing → final product

The goal is not simply to choose a material labelled “PHA”, but to build the right PHA material system for the application.

The most important lesson: PHA is not one material

The PHA family shows how much influence molecular structure can have on the behaviour of a polymer.

Small changes in monomer type and composition can affect how polymer chains move, pack, crystallise and melt. Across thousands of repeat units, those differences can lead to major changes in stiffness, flexibility, toughness, shrinkage, processing behaviour and final product performance.

That is why understanding PHA requires looking beyond the acronym.

PHA is not one polymer. It is a toolbox of polymer structures that can be selected and formulated for very different applications.